My Busa is stock for the most part, the prior owner put a 200/50 rear tire on it but just as the others have commented, my Busa turns just fine.
Low speed turns - The turning radius of any bike is proportional to the ability of the rider. Yes the Busa has a longer wheel base but you can still pull off a tight low speed turn if you're not too scared to lean the big girl over. It's all about leaning, shifting your weight, rear braking and working the friction zone of the clutch.
High speed turns/twisties - Just as with the low speed turns and as with any other bike, you gotta have the b@lls to lean the big girl over and she'll handle the tightest of turns. Tossing her side to side in the twisties is just a matter of pushing the bar and rolling with her. Yes she is a longer and heavier bike but if you tell her to go you better be ready because she is gonna go. Some folks like to lower the ride height or put a phat tire on the rear, that's great for straight line hauling a$$. But, lowering changes the center of gravity and a bigger tire is heavier which changes the gyroscope effect, both of these "cool" mods will make any bike harder to turn.