i am currently running the pirelli diablo sport tires in the stock sizes. even being stock sized, the portion that contacts the road is noticeably wider than stock.
i haven't tried any other tires on this bike other than these and the irc's that my bike came with. the diablo sports came highly recommended to me so i figured i would give them a shot. sorry for the cliche, but the difference is night and day.
i was tempted to go wider than stock, but i was talked out of it. i was told by several people that if i went wider, i would be upsetting the 250's greatest asset.... it's handling. in my struggle of performance vs looks, performance won out.