2x 16GB Kingston KSM26ED8/16ME (32GB DDR4-2666 ECC) @ 1333Mhz stock settings:
@ 1800Mhz:
almost 33% faster 
BUT, it seems like ECC is not properly working and is not reporting when corrections happen. I’m still investigating this a bit further, but I will make an extensive post on this issue soon in ASRock Rack X470D4U2-2T
This is quite a big issue, especially when overclocking, because you actually can’t know if it is really stable (it could be that unreported ECC corrections make it “seem” stable, while it is actually not!)