There was an article in motorcycle sport & leisure about a group of GB500 owners in Yorkshire who all reckoned that a 33 back sprocket gave 'a more relaxed ride and makes all of the gears more useable on the road'. Can't say that I've had much problem with the standard set upon a solo bike, but as GC says, B&C are probably you're best bet. I'd be interested to know what it's like.