Well, I wasn't thinking about the "venting", since it doesn't really "vent" but more uses that path to atmosphere as a reference pressure so that the setpoint remains constant. What I was thinking is that the reg. filling to less of a pressure differential may result in increased efficiency in its operational parameters. Additionally, I think perhaps with the large plenum the pressure "loss" while firing in the plenum will be less due to the approx. 30 to 40% larger volume from a stock gun without the plenum. This could result in a more consistent pressure across the firing valve resulting in a more steady laminar flow and increased efficiency. But it could all also be BS and an entirely different reason.

But I do know for a fact that the plenum increases efficiency without changing anything else on the gun, and I'm sure Ernest would back me up on that...