RE: Zenoah G62 with a 24 " prop???
Depends a lot on the muffler you use. Just as an example here are the results recorded using different exhaust systems, and turning a MenzS 24x10.
Standard box type muffler. The one with the 3 slits in the side: about 4800-5000rpm
Toni Clark stainless steel muffler, wich is the one I use now: 5800rpm
No muffler: 6200rpm
Tuned pipe: 6600rpm
So you see that from one extreme to the other there is a difference of about 1800rpm which is a LOT on a 24" prop. My guess it that with a properly set-up can you will get similar results as running without muffler.
For reference my G-62 spins a MenzS 22x10 at about 6600rpm which is not much for this engine. A lot would be gained by using another exhaust system.