First I think you need to better confirm or demonstrate your bottleneck.
Try benchmarking the 9900k at the following settings:
9900k @ 5 ghz
9900k @ 4.5 ghz
9900k @ 4 ghz
9900k @ 5 ghz (with overclocked ram if not already tested)
9900k @ 5 ghz with hyperthreading disabled
9900k @ 5 ghz with half of the cores disabled (start menu --> msconfig --> boot tab --> Advanced options --> Number of Processors = 8)
The goal being to figure out a few things:
1 - Are you really cpu bottlenecked?
2 - If so, is it because of clockspeed?
3 - At what clockspeed does bottlenecking start/end?
4 - Does having fewer cores hurt performance? This might indicate that more could help.
5 - If not bottlenecked at 5ghz, what is the minimum needed to not bottleneck the gpu.
You may have done some of these tests already, and simply didn't mention them, but if you are already correct in your diagnosis of being a cpu bottleneck due to single threaded performance, than the answer is no amd cpu will be better than what you already have, maybe upgrade/overclock ram or explore higher end cooling.