A guy here runs a 7720 Titan 2 with a 843. He likes it except that on decent corn he has to stop and back up and dump when opening up, because the bin isn't big enough to get through some fields, depending on yeild. Last year I ran cart for him for a few days and he had another guy there with a little older non titan 2 7720 with a 643 and they went almost the same speed, so the 8 row was getting a little more done. I think they were going right at 3 mph, and the corn was doing around 120 bu.
Conversly I've run a 643 on a 9600 a lot, and you can go way faster than I really want to.