To give you an idea, I scored 9931 on 3DMark Firestrike with my RX480 8GB compared to 7391 with my R9 380 4GB. Which is an increase of 34.36% pretty good!
(Intel Core i5 4670k @ 4.2Ghz with 16GB DDR3 1600 Mhz)
I seem to be able to run almost all my games so far on max and get 60+fps on 1080p resolution. (I do not have witcher 3 to test with )