I believe the change in rpm is about 200 rpm per 1" pitch change (inverse relationship - 1" increase in pitch = 200 rpm less in WOT performance). Material effects it as well (stiffer will decrease rpm more). 2000 rpm difference would still require a ton of change in pitch (10") which doesn't seem possible. If you should run a 21-23" I would be surprised if the pitch is actually as low as 13" on that drive. Is the prop too small in diameter as well? I would go to a shop with the prop in hand and the model of the engine and ask them to take a look. Sounds real weird.