i've seen some show bikes that have 8.5 in their rears with a 240 tire. i think you could go wider. you just need a lot of money to do it.
I like to corner, my bike has the OEM spec sized tire, 190/50r17. Actually thinking about downsizing to a 180/55r17 so it's more flickable due to the narrower and taller tire. It ruins the point of a sportbike by putting a wide tire (like a 240+) on the rear unless you're going to the drag strip.
I could see doing it on custom choppers that way it adds extra stability but all a wide tire does is diminish your cornering ability. Plus, you're not going to be taking corners at higher than recommended speeds on a chopper.