Where did you pull those figures from? I made the switch from a 2500k to an R7 1700x and it was faster with the same gtx970, 4.5ghz vs 3.8 GHz. About 14% faster.
Also AMD is cheaper so you can invest on a better gpu. So the benifit is two fold. When I replaced the gfx to a 1080ti I got another 68% increase, bear that in mind.