I have a friend who went from a M 2 Gleaner to a 1640. He said the 1640 had more capacity in beans and about even in corn. My guess is that you will pick up very little capacity with the 1640.
I have very good 850 with 2900 hr,that I would sell. Hydro, airfoil chaffer and 3 screens, new concaves and rasp bars (2 season ago),pictures available, and grain loss monitor. . Field ready! If you are interested Email me and I will call you.