I guess I will just lay out my questions...
1. Which pci-e SSDs can be booted from?
2. Which motherboards (chipsets) support booting from a pci-e SSD?
3. Does Windows 8.1 Pro support it? Windows 10?
4. Would it make any sense even if I could? Is it actually faster than booting off of a SATA III ssd?
http://www.tomshardware.com/forum/280857-30-booting-pcie
Based on the age of this forum post, I would assume that it is possible. Just Google your motherboard and desired PCI-E SSD to find any information about booting.
I'm currently using an 750 Series SSD ( PCI-E ) type, I could have used NVME which my mobo supports but I like the look of the PCI one's better, plus it fit my configuration better. I don't notice much difference in boot time between this and my last OS I had on a 850 PRO, maybe 1 or 2 seconds quicker. I usually keep my PC on 24/7 so it's not really a factor for me though :D
I have an old Revo3 card that supports booting an OS off it, you just have to install the drivers during the Windows install. There's a few that work natively with Windows for booting.
Might be (probably is) apples to oranges but all of my drives are running on an SAS controller card through PCI-e, which includes my boot drive and I never had any problems on my GA990FXA-UD3 board. I would hazard a guess that PCI-e boot support is up to the motherboard rather than the SSD/HD, it just becomes a matter of making sure the boot manager for your OS gets installed to the correct device.