As for upgrades, I have Real Flight G5 and am really happy with it.

On your question, the rudder mix is about the worst way to handle that problem. Personally, I'd say that a pilot who hasn't yet learned to use the rudder shouldn't be flying a P-40 at all, but I also understand that sometimes you get planes you like and don't have money for anything else. Since you have a sim, why not just work on coordinating your turns? New skills is what it's for, so use it. While you're at it, work on learning to use your rudder to place landing approaches where you want them and to cross control for wind.

If you absolutely must band aid your adverse yaw problem instead of fixing it with piloting skills, aileron differential is a much, much better solution. Try setting them up where you have about 1/3 more up than down and go fly it. My Cub likes roughly 3/4 more up than down to drag its tail through a turn without me having to get on the rudder, but Cubs have a much worse adverse yaw problem than P-40's.