last time I checked, the average difference between RX 480 and GTX 1060 (with the latest drivers) was next to none. Far from 12% on release. It is a shame that it took AMD over 6 month to get their drivers right, but we are now, so who cares as long as it works.
In real world it translates to 1-2 FPS which is really unnoticeable. And of course there are some games that clearly prefer the AMD HW (and vice versa). but we care about avg more.
Unlike GTX 1060 3GB, the RX480 4GB is fully enabled version and the difference is really just the amount of vRAM.
For 1080p gaming, there is 0 performance benefit of over 4GB vram. and frankly, those cards are not meant for higher resolution.
So the RATIONAL choice is getting RX 480 4GB as it is significantly cheaper.
IMHO, the choice between RX 480 8GB, RX 480 4GB and GTX 1060 6GB is very clear if the pricing is right. The GTX 1060 3GB is slower and cost the same or higher than RX 480 4GB. makes 3GB irrelevant.
Right now, the rational buyer list is:
1050 - for low budget.
1050Ti - only low profile version for small form factor.
RX 470 4GB
RX 480 4GB
GTX 1070
GTX 1080