You won't gain any substantial perfromance buying a 10900. Its all about clock speed. You can save a few bucks buying a 9900K and overclock. Air cooling has improved greatly if your not into water.
There were improvements in the 9900 that makes it a solid chip. I use mine for cad as well as some editing so it made sense for me. As a dedicated DCS machine, I did see improvement over my 4790K of course, but not as much as you might think. A SSD is far more important than the latest chip for load times. If you can get high Ghz RAM, it'll help with high load missions.