I would likely cut off some auger flighting considering you have a wide body and the auger flighting can bring a lot to the middle.
I don't know what is standard and what isn't anymore on newer machines. Does a 780 come with the rotor intake flow kit standard, not a big improvement either way IMO. Does it have the new Claas knife kit or whatever they call it below the back side of the impeller, between the rotors? The knife that I built is the best thing I have installed for impeller improvement. I have only had material build up above the impeller once since, and I think there was a small tree that came through to start the build up. I have only plugged impeller once since installing also, in some real nasty weeds after the sun went down and I was driving too fast with cylinder too slow.
Are you actually plugging the impeller, or just getting material built up between impeller and tank, and that material rubs all day on the impeller in the middle, wearing it down?
I am highly considering taking a pvc pipe, about 12" in diameter, and cut it in half length ways about 2 feet long. Take it and bolt it above the impeller in the mid section. This will prevent any long, green material that comes around the top from hairpinning in the cavity on the underside of the grain tank.