I have a 8200 and am pulling a 13 shank anhy machine.  I judge what gear to run in by trying this.  I can pull it in say 7th at 2100 rpm and 4.5 mph.   I shift up to 8th or even 9th and if I can run the same speed, 4.5 mph @ 1900 rpm, or what ever it might be, I figure I am OK, but when the pulling become more difficult and the rpm drop down to 1800 or 1750 for a period of time what have I gained by running in 8th or 9th.  Nothing.  I would be better running in 7th at a higher rpm and I have more ability to pull through those hard spots with a more constant speed.  I don't see any advantage to shifting up and having not lugging ability left.