R9 390x bottleneck i5 4570?

No, the % difference would be the same of course. But 100 to 115 is 15%, so in the case of 30 FPS being the old 100, it would be 30 and 34,5 FPS instead, no big deal to be honest. The FPS is going to fluctuate so often that you won't notice a difference while playing, just benchmarks, or if you're obsessed at looking at the FPS counter.