My OS 160 ran up to the low-mid 9k's with APC 18X6. The APC's are thinner airfoil & less chord plus the curvature at the tips must be more efficient than the Zingers. I discovered that my Saito 150 was swinging an APC 17X6 1K more RPM than Zinger 17X5 wood-go figure?! I would think it would be the same with a 2-strk. You are right about the break-in too,it will take some time for it to open up & lean out. I just remembered,another guy at my field had MA 18x6 Fiberglass Classic on his OS160-was running almost 10K with that paint stick! The tips were going super-sonic,made a racket you could here a mile away. Another go figure?!