A 1/4 end mill should clean it up really fast. You will be making really shallow passes so you can move pretty fast. I would guess 15 minutes or less. My beds have been pretty close as built, and I usually end up surfacing my parts or cutting all the way through so extremly flat has never been an issue for me. Maybe I should give this a go as well it can only help right?
No experience with the T Slots sorry.