Mitch, have you thought about trying an exhaust ball joint instead?
The two parts are held together by spring pressure, and they seal quite well.
Best part is that they can be adjusted for slightly different frames by hand, which could have eliminated some of the problems with Q pipes and AAEN pipes fitting up to other frames.
One of the problems with any type of straight slip fit joint is that any side load (torque, vibration, poor fitting pipe, etc.) will cause a gap for the exhaust and oil to shoot through back onto the engine, whereas a ball joint would just rotate and maintain a seal.
I've been thinking about converting a pipe just to test out the concept, I'm tired of using O-rings and silicone.
The trick with a ball joint is to have the inside ball coming from the engine and the socket going to the rest of the exhaust, that way the ball will expand from higher temps and seal tightly against the socket.
I guess the only problem really is that the small chamber it creates might change the tuning of the pipe itself...which could be remedied if you were building pipes, obviously.
This is about the closest picture I could find of the type of ball joint I have in mind, they're on nearly every piston engine aircraft flying today.