The QJ air door (secondary air valve) rate can limit the total CFM on the back side if the spring is tight enough, but it also controls the secondary metering rod position (on a Holley, the diaphragm can be anywhere, and the PV still opens when you want), so if it's tight the motor gets less fuel. Not opening to 90° also creates a flow bias which will affect front vs. rear if the carb is positioned with shafts across the engine (like on a V8).
A TQ works pretty similar, but has an extra tuning headache: the air valve linkage is part of the choke linkage, and getting them both to work (on a street car) took several hours (Chrysler 360).
A 250 would have to turn pretty fast to get an improvement with a 650. Assuming 95% VE, even 7,000 RPM needs only about 529 CFM. If I were trying a 650 here, I'd run slightly smaller high speed bleeds to compensate for the lack of WOT vacuum rather than rely entirely on PVCR changes.
