Coming from a wealth of time with AMD FX, it is not worth the extra 100-200mhz you are going for.
Drop cpu multi to 20, and save some heat and voltage.
Your clock frequency drops are likely directly due to VRM's that are not happy with the power draw or temp.
Remember, the cpu socket bleeds off a TON of heat throughout the motherboard.
I personally ran 4k with a 1080 TI, and the setup did pretty well, mostly because of running ram beyond 2400mhz and northbridge beyond 2800mhz.
Whenever I have ram set to 2133mhz or slower, and northbridge at or slower than 2600mhz, the entire computer feels very sluggish and pathetic, after having it running near-maxxed out for so long.
Even my X99 setup with its 5930K can beat it at stock clocks and making use of the X99's quad channel memory, even at JEDEC 2133mhz.
If you intend to maintain the FX rig, find a way to keep the VRM's and northbridge as cool as possible, and crank the **** out of the IMC/northbridge speed. 2800mhz is great, but FX gets another speed boost if you can hit 2900+Mhz. It takes a butt ton of voltage though, so use it sparingly, as I have personally killed an old 8120 very recently while trying to crap out some high numbers. I did manage 5.2ghz on it though, with an FSB/ref clock of over 350mhz