Don't use a low pitch prop, you need to load the motor, so 6" pitch prop or more.
It takes less shaft power to fly the airplane with a 6" pitch prop than a 4" pitch prop, substantially so. That sounds like less load, not more. Which is also why it works at all - you are trading a much less good prop for unloading the engine so it can respond more in flight. The fact that you lose some power is moot, because the power is still more than sufficient.
I would also note - the only perceptible competition success with a 4-stroke were with props 4.5" at around 10,000 rpm - like the 2003 NATS. The ultra-low-rev approach (first used in this country by Igor Panchenko as far as I can tell) came later.
The Berringer-style venturi is a *great* idea that definitively solves one of the biggest problems - trying to adjust the speed using the mixture - by allowing you to change the choke area instead. One day, when I am retired and have some time to fiddle, I might make something similar for a 2-stroke.
Brett