I am inclined to go with the spark advance idea. If the butterfly position was important, it would probably work both ways from dead straight, which it does not. The Walbro carb is well known for the lack of rpm reactions near full throttle, so the reason must be somewhere else.
I usually set the trottle stop arm 5mm (7/32" ) from the stop at full throttle. On EI, This gives the same best power, and better throttle response to transmitter inputs.