I have a 350 (started out as a ZZ3 crate motor) with a COMP 274, Torker II intake, and a 650 dp. I agree with everyone else, it's in the tuning. I got 360 hp from the engine with this set-up naturally asperated and over 500hp with a supercharger.
rrent, a vacuum secondary card will almost never kick in as fast as a double pumper. I can punch the throttle at idle (900 RPMs) and open all four barrels all the way. The typical secondary spring won't begin to open the secondary throttle blade until around 1,600 RPMs. BUT, you are correct that the engine may not need all of that air and it takes major tuning to get the dp working right.
Hugger Orange & white 69 Camaro with supercharged 350, Tremec TKO, and 3.73 12-bolt