Here's the result of my quest...
I have the Intellimouse Explorer optical mouse as well (though I have kept that just for browsing around and have since switched to the Logitech Wingman Gaming Mouse for UT). I had my IE mouse settings cranked to the max in Windows with no acceleration, and cranked to 18 in UT, but my mouse was enormously bad in UT. It was as if the snap was on, a diagonal pull was extrememly jaggy. I couldn't figure it out for the longest time. It finally was revealed to be the Windows settings. By reducing the sensitivy in Windows to one notch left of center and going with maximum acceleration, I was able to get rid of the jaggies in-game. However, my accuracy in lowgrav instagib was still very hit and miss.
Fast foward to a few weeks ago. I've switched to the old logitech gaming mouse (the grey one) for UT and was talking with my Fireteam leader, Fragmaster{RoA}. He mentioned that his Windows settings are the same as mine, now, but he has his UT mouse settings set to 3.5! I thought that was phenominally slow, but tried it myself. Af first, it was very difficult to get used to, but, lo and behold!, my accuracy took an immediate jump, doubling my normal accuracy. I've redeveloped the flick move (something that wasn't necessary with UT 18 mouse settings) and have been much more accurage since then.
FWIW...