I use 850 nuts on my 750 with the Suzuki exhaust washers (1 per side). I have slighty less than an 1/8" to spare. Big fins, better cooling, nicer crome.
As to the threads on the nuts, I believe that the casting of the head 750 and 850 are basically identical and the larger bore of the 850 leave less area for internal threads.
As far as the clearance from the balancing pipe, my arguement would be that the 850 nut is much thicker and wider with much more mass then the 750.
My 2 cents, some one set me proper.
As far as tightening, don't be puttin a breaker bar or nothin on it, snug them down real good after every run for 3 or 4 cycles and check once in a while. Don't go crazy with it, it's a feel thing that you will get.