It just occured to me that one thing that could be causing your problem. Have you checked to make sure the that when the gas pedal is on the floor that the primary throttle blades are fully open? If they aren't it will also restrick how far the secondaries will open.
It's amazing how much difference even the being 10 degrees away from fully open will make. I learned that when i swapped on a Demon carb to try and the car ran like a big. Checked the throttle blades and they were about 15 degrees away from fully open.
