RR24 I'm not sure what your altitude is there but guessing its low from the turns that your getting with the 18x8 I'm sure your correct in thinking that you will need another prop, Jody has been using a 19X8 Xoar Laminated and getting about 7300 with the stock exhaust. Just remember you will gain approx 800-1000 turns in the air.
I bought the 18x8's for myself but i'm higher and know for a fact I won't see the turns you lucky guys at close to sea level get POUTS. That little 30 is a potent little engine and light to boot.
I have been so busy here I have not even drilled one out let alone be able to run it.