Will the Athlon x4 860k bottleneck HD 7970/R9 280x?

Solution


Unlikely. The only benefit would be two more Piledriver modules, but that would only give you one more FPU, two more integer cores, and one less decoder. The single threaded performance will be 5-15% slower than that of the Steamroller cores on the Athlon 860K, which is much more important for gaming performance in DX9/10/11 titles (all modern games).
Your bottleneck depends on the specific game engine, and what detail settings/resolution you are running. Many games will run just fine with that CPU, but there are some that will lag some in certain instances. Overclocking/ tuning your hardware will help.
 


Unlikely. The only benefit would be two more Piledriver modules, but that would only give you one more FPU, two more integer cores, and one less decoder. The single threaded performance will be 5-15% slower than that of the Steamroller cores on the Athlon 860K, which is much more important for gaming performance in DX9/10/11 titles (all modern games).
 
Solution
I would not switch boards and cpus to another amd combo at this point. I only say that because of their supposedly releasing new chips this year sometime. After that is say you can make the decision. But I think what you have vs the 6300, you probably won't see a lot of difference in performance.