You’ll never get the seamless precision from a controller that you do from a keyboard/mouse. If Microsoft thought it possible for XBOX players to compete equally with PC players they wouldn’t have isolated them on XBL.
Interesting fact: Halo2 PC was designed specifically to play with Halo2 XBOX on XBL. Right before release it was switched to a PC-only XBL server due to complaints from XBOX players.
Interesting fact: Games for Windows LIVE! is very similar to XBL and was originally an identical port. While signed in to Windows LIVE! you can interact with XBL players as though you were connected to XBL.