Realistically Navi is going to have at most half the memory bandwidth of VII (Unless they go for an as yet unseen 384-bit GDDR6 bus and even then it's only three quarters the way if also using so far unseen 16Gbps memory) and I doubt they could improve their compression techniques much to allow them to throw in that much more compute power without being bandwidth starved, I'd guess about 75% as fast as VII is the best we should expect if they're going sort of all out(For this years cards).