The 750/850 may be faster with skinny tires. Certainly, their weak, old frames would never be able to resolve the stresses created by the enormous traction produced bu "fat" modern tires. It would result in wild front end oscillations that make the bike uncontrollable. Modern frames and suspension can leverage these benefits, where old bikes simply can't - due to their design.
I think if you take a modern fat tires 600cc supersport, and compare its handling at speed to an old Manx it will be an eye opener.
Ok, you may have to rev-limit the supersport bike to 7,000RPM, where it makes 50HP, like the Manx.
The fat modern tires are needed not just to put the power down to the ground, but also to stick the bike under high speed cornering, and breaking forces. A Manx is a good old bike, but they never generated those types of forces like modern machines do.
Yes, the 180 section rear tire on the 961 is gross overkill for the 70+HP, but the same tire provides excellent cornering traction, and the 120 section front tire allows the same, and also sustains very high braking forces. With the suspension sag set properly, front and rear, and proper tire inflation, and wheel alignment, I think a 961 would avail itself well against an old Manx, or Commando for that matter.