I wonder if there's not something wrong with the Quadra. I have a US-41 (basically same as Quadra) and use a 20-6 prop; on pump gas at 40:1 with Klotz, it will turn 7500 rpm and pulls a 22 pound airplane around really well. There's two BUSA D-VII's in my club, one with a G-38 turning a 20-6 prop and he can do all kinds of scale maneuvers. The other D-VII has a G-26 turning an 18-6 and while it's no speed or aerobatic demon, it will fly okay.

Have you got the Quadra leaned out, or has it got a lot of time on it? Strange that it doesn't do better than it's doing.