I did a quick test last night using the Xbox and ESPN NFL 2k5. I turned off 480p, 720p, and 1080i in the video settings, forcing the Xbox output to 480i. BTW, I'm using a component connection. 480i was confirmed as the input format using the 'info' button on the Mits. Then I went into practice, and practiced my kick-offs. Repeated the practice, this time with 480p enabled. My results: Absolutely no difference in my timing. I couldn't detect a bit of difference in the timing required for me to get MAX power on kicking.
If I get the chance to move the PS2 and connect up using the front inputs on the tv, I'll report what I see then. Hope this helps.