Hmm. I can't directly answer your question, but I can offer this: W7 performs very well on the Mini--so much so that any slight gain in XP would not be worth the loss of polish. I dual boot W7 and Ubuntu, and I find performance to be very similar.
Perhaps someone who is dual booting XP and W7 or switched from XP to W7 (I went from vLited Vista to W7) could better answer your question.
How much RAM do you have? And is SSD
space an issue? Both of those factors could also influence your decision. (Although W7 runs on 1GB, I wonder if XP does better with 1 than W7 does...)