Based on the TH tests of the 7700k the performance difference in minimal, under 10 percent. And because a motherboard has little to do with the performance of the processor other than features and the ability to overclock, there is no reason to get a Z270 unless you are building a new system. You won't see any performance increases there.
As for ASUS' product line, thats a question for them, I would imagine a Maximus IX to be a safe bet though.