Anybody go from performer to an rpm air gap

Dave. The unmodified notch is fairly small. You're probably right that it helps balance as the secondaries open. Pressure would certainly even out left to right, and one would expect some, but not a lot, of fuel-air sharing.

Attached is a picture (hopefully) of an LD-340 with the divider notch pretty much original. (Yes someone, not me, made the exterior into a stealth version).

You'll see the LD-340 design really looks nothing like Performer RPM. If I ever get the chance, I'd like to try the RPM as well as a Holley Street Dominator. I had the Holley on my last engine with a somewhat similar cam (but not quite) and I don't think it was any worse at idle and low rpm than the LD340. Take a look at those big runner entrances - my theory is that the LD340 was a good higher rpm dual plane and that's where its rep came from. My guess is that's why certain people say it is just as good as the Performer RPM - they're looking at high stall (3000 rpm) street-strip or strip use.