Those who have mentioned the low pressure in the rear tire on hardtails are right. On my panhead, the manual says the proper pressure for the rear 5.00 x 16" tire is 16 PSI! At that pressure it absorbs a lot of the bumps. No, you cannot use a low profile tire. The rubber and air is what keeps you from getting a bent rim. I would never run low profile tires anyway. They look like something that belongs on a zoom-splat. Yeah, I know, it's a matter of taste. On my Indian I have a 3.75 x 18 on the rear. I run that at 25 PSI.
As I said in my earlier post, I am very used to riding hard tail bikes. Still, I don't think I will change my XS650 to a hard tail. I like the way it handles just fine as is. Those who explained that replacing the rear shocks with struts will NOT duplicate the riding characteristics of a hard tail are right. You will end up with a bike that handles worse than it did with the rear shocks, but without the shock absorbtion. It won't have a proper hard tail seat, either sprung or well padded, so you will just wind up with a bike that is more uncomfortable than a real hardtail, and will not handle as well as a real hardtail. Still, if you can take that, you can be sure that you will be able to tolerate a real hard tail. In fact, after struts, you will find it a GREAT improvement.