Following this thread is interesting. I also play inverted and find it much easier to aim, although I can do fairly well with it on normal as well.
Practice is the only way to get good. I have my roots in PC gaming as well, but it just takes time. Like somebody else said, don't focus on it, but just play the games and it will come naturally.
Also, I agree about the KB/M adapters (XFPS, FragFX, etc.) You will just not get the same experience as a PC FPS. It is jerky, stutters and has deadzone and sensitivity issues. You can adjust it to make it playable, but I had the SmartJoy Frag for the original XBox (same thing) and it was very difficult. I went back to the original controller after a week or so.
The problem lies in how the games are designed. They are not designed to be used with a keyboard and mouse, but with the gamepad's analog thumbsticks instead. They use a different control scheme, so when you plug in one of these keyboard/mouse adapters, the mouse is really just mimicking (sp?) the analog stick movement, which it doesn't do very well.
Also, the price just doesn't seem to justify it. For the $80 + $30 for a wired controller, you are investing over $100 just to attempt to play FPS games as a PC would. For that same $100, you could get a good gaming mouse, mouse pad, new keyboard and still have some change left over for a game.