AVS 2 CFM???

The AVS carbs, Carter & Edel are mech sec carbs. All four barrels open when you go to WOT via mechanical linkage. The air valve in the secondaries [ Air Valve Sec, AVS ] controls secondary air flow.

The 800 & 650 have the same size secondaries. The primary bores in the 650 are smaller & that is the difference. It is tempting to go bigger, but your cam is pretty mild & I doubt it would allow the 800 to use all of it's airflow. The smaller primaries of the 650 will also have better throttle response for street driving & for these reasons I would pick the 650.