More speed, means higher difficulty and higher electricity consumption and the same income. Because most people use claymore for zcash, so everyone will move to new miner. What is the point?
How about that he want to be competitive and beat other miner? It's fair. One other thing is for sure RX cards can surelly hash higher.
I expect around 350 h/s for 390x, 300 h/s for 290/290x and minor improvements for 280/380 (maybe 5%+). Little to not improvement for 470/480 due to the narrower memory bus.
Don't get your hopes up too much. The interesting part will be power draw here, I am not interested in 15% hash rate for 25% Power draw that's for sure.
Exciting times still.
Edit: Heh, i was off quite a bit.