Will it be a bottleneck? Yes, it will. How much of a bottleneck, though- in some games it will be bad, some fine. At higher resolutions and using DX12 it won't make so much of a difference. You probably won't benefit from going to an FX-8350. You might see a benefit from going to an i5, but is it as much as the benefit from getting an R9 390X? Unless you are doing heavily CPU dependent work/gaming the R9 will improve performance more than an i5.