I have sealed the muffler against the crankcase, and the two muffler halves, with hightemp silicon on some engines and that will help a bit with all the oil.
With the stock muffler, 10% nitro (and 20% all castor) it will spin a graupner 6x3 prop at around 14500rpm, so it sounds like your engine is right in the ballpark of their performance.
NoNo Fulton saw a bit higher numbers, but he also had the muffler mounted upside down(!):