this is a great thread,thanks for starting this up again mike.this is just my opinion but i believe the most important thing for igniting bh209 is a trully sealed breech with primer being held firmly in place with no play.two of my knights have knight NFPJ plugs that have convex ends 1/8" flash channels and i believe .030 flash holes and have worked flawlessly down to(if i remember correctly)11*.one of these plugs fit great and is 99% blowback free,the other had a little play so i used a very thin O-ring that busta sent me(thanks again)made that one 100%.my knight shadows(i,ve had two)with the same BP design with the addition of the proper sized O-ring work good too but only for 8-10 shots which is still fine for hunting.my knight vision NFPJ and triumph both came blowback free and have never had any issues.i have no doubt that a larger channel or flashole helps but some but some plugs without a flash chamber at all (knights crossfire-metrics unlimited for the huntsman/sidekick)work very good too.just my thoughts......karl
From my quick first initial read of your post you have the bases covered.
The only thing i would add, and you can confirm with Busta, is if the flash channels were opened to .159" you would improve the breathing and the handling of the gas emitted by the various primers. Then along with a .032 flash hole you would also reduce the amount of fouling being built in the flash channel.
My factory DISC breech plugs will shoot BH fine as long as I keep the carbon out, they can get dirty but they still shoot even with a FPJ.