Even if you do use a keyboard and mouse for XBox, it would be limited. You can't turn your point of view on the XBox like you can on a PC. The turn speed is limited to as fast as the controller can turn it. So, if you're thinking that you'd be able to make the quick, reflex movements with a mouse, then you're wrong.
Having said that, the controls for Halo2 are pretty tight. It took me about 10 minutes to get used to the controls. Acutally, I'm more precise in Halo2 than I am in UT2k4. If you play me in UT2k4, you know I'm a spammer because I can't hit a think with hitscan. In Halo2 on the other hand, sniping is by far my strengths. I can hit running, jumping, or close range head shots all the time (granted, some of this is due to the slower game speed).