Sounds like in order for the Infinity Fabric to run 1:1 with the memory, the cap is 3733 Mhz. After you get beyond that, it drops to 2:1, so you get faster memory speed at the expense of a slower Infinity Fabric link. Probably the sweet spot is going to be 3733 Mhz.
So far, as far as I know you don't really see much in the way of performance improvement with existing platforms today going too far past 3200MHz in games. Its likely that 3733MHz will be the new ceiling for Zen 2, if we even need to go that high. If memory compatibility is as good as AMD claims, I've got some 4,000MHz modules here so we'll see.