Well you can pay extra for the 9900ks, or you can get the 9900k which in most cases easily runs 5 GHz on all 8 cores with no problem as long as one has a decent cooler.
Currently and at least for the near term future as you know IL-2 is very much dependent on CPU Horsepower.
I do not see any advantage at least first glance of the newer 10th gen chip for just what we do here.
Now if the new chips were to gain one say an additional 500 MHz plus then it would be a different story. Of course there may be other benefits with the new chip architecture and the motherboards required for it that are not known yet.
I also would not disable any cores. Hyperthreading yeah, I disable that . Heck you pay for the cores, let the game use them. Plus there are a lot of other processing threads going on outside the game that needs to go somewhere.