Just this past weekend I had the opportunity to witness a fellow club member fly his OS GT33 powered EF 78" Extra on muffler. His was turning an 18x10 wood prop (didn't get the make) at about 6600 rpm. It was a new set-up and wasn't fully tuned but I seriously doubt he would get more than another 100-200 rpm out of the muffled engine.

The OS33 will turn the same prop at +1000 rpm if it were piped.