Don't expect too much for the custom RX 480 cards, the tests show it's vram bandwidth bottlenecked as performance scaled very well with just memory OC.
Unless custom cards can get GDDR5X, even IF they clock it to 1.6ghz, I don't expect performance to scale much beyond 1.4ghz.
1.4 ghz would put it above the 980 at 1080p and just under fury. That's quite good imo.