The XS650 also had the same power and more torque than some of the 750 twins of the same day..................It's not necessarily the bike. A bad bike ridden by a good rider will out perform a good bike with a bad rider..............the Early XS650 70-73 was considered the pig of the day................Here in Australia we used to have a 6 hour production bike race..........Production bike races are bikes off the showroom floor and maybe a allowed an engine blueprint but basically the same bike as a punter will buy...............in 1973 2 XS2's, after 6 hours of racing against Nortons,Triumphs, Kwakas, Honda's, Suzuki, and other Yamaha's with, 4 cylinders, 3 cylinders and 2 strokes. the top 10 placings were 1st, Honda 750/4..........2nd Yamaha XS2 ....3rd, 4th, 5th, Honda 750/4........... 6th, 7th, 8th, Honda 500/4...... and 9th a Yamaha XS2.
After 6 hours the Honda 750/4 had lapped the XS2 3 times...........Sounds a comfortable win.............The distance the Honda traveled was 401 miles and 333 laps of the 1.29 mile circuit..........In affect the Honda traveled 4 miles further than a bike that had 2 less cylinders and was 100cc down on size.........
Even these guys on here now, who worked or owed dealerships, wouldn't consider an XS650 and turned their nose up at them in the day...........These guys are on here now, with XS650's and although they recognize their deficiencies in some of the old technology they will ride that bike with more pleasure than newer or the old bikes of the day, and say they would never sell it.
May be you need to do some research on exhaust systems for XS650 Bob. What there hasn't been done, or known by now there never will be.