I have some data with the Zinger 20x6. My Moki 180 would turn this prop at 7500 rpm peak. But your Saito 220 is new. I would think that once broken in, you gain a few hundred rpm, and using 30% nitro fuel will hopefully gain another few hundred rpm. So 8000 rpm w/ a good 20x8 (not Zinger) is not out-of-reach.