Yes and No !
First off, always make sure that your GPU is the limit factor. That means crank up visual settings in games and desktop resolution, do not use anything lower than 1080p with your R9 390 in games.
You will be fine.
However, some games that use old crap engines like Arma III, World Of Tanks, etc, or even really old games like Crysis 1, only run in Single/Dual Thread. In those games you will lose some performance, my guess about 40~50% in the worst scenario.
In my case i do have old single Thread games, like RFactor, GTR2, Crysis 1, Killing Floor, FarCry1, but even so my R9 280X O.C + FX-8350 just does the job.
Does my FX-8350 bottleneck my R9 280X in very old games? Yes it does. What the clock i need to run 100% GPU in old games like Crysis 1 for example?
About 5.0 Ghz as you can see below.
Does that mean i cannot run Crysis 1 with a stock FX-8350? No, i play Crysis 1 perfecly fine even with the GPU at 70~80% load.
My suggestion to you is play at 1440p, and if the game runs smooth, dont bother with bottleneck.
In modern multi thread games like Crysis 3, BF4, DOOM, FARCRY 4 the FX-8350 stock do not bottleneck the GPU.
This ScreenShot is with Crysis 3 and a stock FX-8350 + R9 280X. As you can seem no bottlenecks GPU always in 100% !
Good Luck !
.Edited by jclafi - 6/6/16 at 2:03pm