I also have a 7720 II. One of the best mods I've done is to replace the top chaffer with a fixed air foil. Mine came from St John's Welding, however, I think he may have stopped making them or sold the patent to someone else.
I run corn, soybeans, and wheat. I really like the job it does and was a real improvement in capacity. The increased air and upwards angle have removed this area as a limiting factor for capacity in my combine. I'd stay way from the adjustable air foils. Not sure, but I think Peterson makes a good fixed one now. Good Luck.