This sounds like that JCWhitney "Turbolator" device that essentially did the same thing: had a butterfly valve in the exhaust to regulate backpressure or something.
No one tested it out, but I recall someone had a soda can on their exhaust tip. Not sure how that went -- I think it blew off.
Let's try this -- stick a potato in the tailpipe on startup and see what happens
Is there any method to the madness, Scientifically? Are adjustable restrictors effective?
RH77