There’s already a listing of the IOMMU grouping of this board elsewhere on this forum but not fully populated. I asked ASRock support to send me the IOMMU grouping of this motherboard if all of the pcie x16 as well as all the m.2 slots were populated. ASRock support fully populated a board for me and sent me the iommu grouping output in that case. I did not expect this, I’m really impressed by this level of support.
The short version is: All GPUs in the x16 slots as well as all m.2 slots end up in their own groups. This board seems to be close to perfect. 
For BIOS 2.0:
IOMMU Group 0 00:01.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 10 00:14.0 SMBus [0c05]: Advanced Micro Devices, Inc. [AMD] FCH SMBus Controller [1022:790b] (rev 59)
IOMMU Group 10 00:14.3 ISA bridge [0601]: Advanced Micro Devices, Inc. [AMD] FCH LPC Bridge [1022:790e] (rev 51)
IOMMU Group 11 00:18.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1460]
IOMMU Group 11 00:18.1 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1461]
IOMMU Group 11 00:18.2 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1462]
IOMMU Group 11 00:18.3 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1463]
IOMMU Group 11 00:18.4 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1464]
IOMMU Group 11 00:18.5 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1465]
IOMMU Group 11 00:18.6 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1466]
IOMMU Group 11 00:18.7 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1467]
IOMMU Group 12 00:19.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1460]
IOMMU Group 12 00:19.1 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1461]
IOMMU Group 12 00:19.2 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1462]
IOMMU Group 12 00:19.3 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1463]
IOMMU Group 12 00:19.4 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1464]
IOMMU Group 12 00:19.5 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1465]
IOMMU Group 12 00:19.6 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1466]
IOMMU Group 12 00:19.7 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1467]
IOMMU Group 13 01:00.0 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] Device [1022:43ba] (rev 02)
IOMMU Group 13 01:00.1 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b6] (rev 02)
IOMMU Group 13 01:00.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b1] (rev 02)
IOMMU Group 13 02:00.0 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b4] (rev 02)
IOMMU Group 13 02:04.0 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b4] (rev 02)
IOMMU Group 13 02:05.0 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b4] (rev 02)
IOMMU Group 13 02:06.0 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b4] (rev 02)
IOMMU Group 13 02:07.0 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:43b4] (rev 02)
IOMMU Group 13 03:00.0 Ethernet controller [0200]: Device [1d6a:d107] (rev 02)
IOMMU Group 13 04:00.0 Ethernet controller [0200]: Intel Corporation I211 Gigabit Network Connection [8086:1539] (rev 03)
IOMMU Group 13 05:00.0 Network controller [0280]: Intel Corporation Device [8086:24fb] (rev 10)
IOMMU Group 13 06:00.0 Ethernet controller [0200]: Intel Corporation I211 Gigabit Network Connection [8086:1539] (rev 03)
IOMMU Group 14 08:00.0 Non-Volatile memory controller [0108]: Intel Corporation Device [8086:2522]
IOMMU Group 15 09:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Redwood XT [Radeon HD 5670/5690/5730] [1002:68d8]
IOMMU Group 15 09:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Redwood HDMI Audio [Radeon HD 5000 Series] [1002:aa60]
IOMMU Group 16 0a:00.0 VGA compatible controller [0300]: NVIDIA Corporation GT200b [GeForce GTX 285] [10de:05e3] (rev a1)
IOMMU Group 17 40:01.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 18 40:01.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 19 40:01.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 1 00:01.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 20 40:01.3 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 21 40:02.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 22 40:03.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 23 40:03.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 24 40:04.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 25 40:07.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 25 40:07.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1454]
IOMMU Group 25 45:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc. [AMD] Device [1022:145a]
IOMMU Group 25 45:00.2 Encryption controller [1080]: Advanced Micro Devices, Inc. [AMD] Device [1022:1456]
IOMMU Group 25 45:00.3 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] USB3 Host Controller [1022:145c]
IOMMU Group 26 40:08.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 26 40:08.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1454]
IOMMU Group 26 46:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc. [AMD] Device [1022:1455]
IOMMU Group 26 46:00.2 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] FCH SATA Controller [AHCI mode] [1022:7901] (rev 51)
IOMMU Group 27 41:00.0 Non-Volatile memory controller [0108]: Samsung Electronics Co Ltd NVMe SSD Controller SM951/PM951 [144d:a802] (rev 01)
IOMMU Group 28 42:00.0 Non-Volatile memory controller [0108]: OCZ Technology Group, Inc. RD400/400A SSD [1b85:6018] (rev 01)
IOMMU Group 29 43:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Oland PRO [Radeon R7 240/340] [1002:6613]
IOMMU Group 29 43:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series] [1002:aab0]
IOMMU Group 2 00:01.2 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 30 44:00.0 VGA compatible controller [0300]: NVIDIA Corporation GT200 [GeForce GTX 260] [10de:05e2] (rev a1)
IOMMU Group 3 00:01.3 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 4 00:02.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 5 00:03.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 6 00:03.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1453]
IOMMU Group 7 00:04.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 8 00:07.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 8 00:07.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1454]
IOMMU Group 8 0b:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc. [AMD] Device [1022:145a]
IOMMU Group 8 0b:00.2 Encryption controller [1080]: Advanced Micro Devices, Inc. [AMD] Device [1022:1456]
IOMMU Group 8 0b:00.3 USB controller [0c03]: Advanced Micro Devices, Inc. [AMD] USB3 Host Controller [1022:145c]
IOMMU Group 9 00:08.0 Host bridge [0600]: Advanced Micro Devices, Inc. [AMD] Device [1022:1452]
IOMMU Group 9 00:08.1 PCI bridge [0604]: Advanced Micro Devices, Inc. [AMD] Device [1022:1454]
IOMMU Group 9 0c:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc. [AMD] Device [1022:1455]
IOMMU Group 9 0c:00.2 SATA controller [0106]: Advanced Micro Devices, Inc. [AMD] FCH SATA Controller [AHCI mode] [1022:7901] (rev 51)
IOMMU Group 9 0c:00.3 Audio device [0403]: Advanced Micro Devices, Inc. [AMD] Device [1022:1457]