MLA, I don't know much about 7720's but I have an "air-flow"(jobber) top sieve in my old 914 and I don't like it. I can't get enough wind up through it. Maybe the Deere combines have a better fan than the IHC.
Based on that I would say stick with stock or at least stay away from the air flow sieve. The one I have cannot be adjusted!!
I've a littermate to your 7720, and I switched to a St John "airfoil chaffer" about 10 years back. 1) Price stock - insanity (unless Deere has responded to aftermarket pressure). I cut mostly corn and wheat. I can blow wheat and milo out the back, but I run wind wide open on corn. I've never seen the shoe overrun by corn - you know you must have all material "live" on the shoe because if it starts sitting or resting on the shoe, you are seeding not cutting. Elavators have begun including foreign material in their receipt, and my bin samples are exemplary. Some of the new Deere conventionals are sold with the aftermarket chaffers.