Lunati Voo Doo Cam.

Some chamber polishing might help. I'll venture to guess that no quench and the slower overall burn rate in the open chamber are probably what is limiting the CR for you. (I also wonder if a manual trans set-up will be less forgiving.)

I was gonna ask about ignition timing but you have addressed that. Have you slowed the rate of adding in the mechanical advance? (I.e., heavier springs...)