I'm gonna sound snobbish, but my badly abused Sportster .20 (with silicone o-ringed head, no shims) turned an 8x6 APC prop at 14,600rpm on 5% nitro 25% castor fuel and McCoy MC59 plug. I think this one has a gremlin or something - the engine review for this model showed about a 1,000rpm lower reading on a zinger wood 8x6. Back on page 17 of this thread is a few pictures of how nasty this thing looked inside when I got it. Maybe it was some speed additive.
These engines are timed really low, so tend not to like running that fast for very long. Loading them down for around 11,000rpm on the ground is close to ideal.