I fly a Gremlin with a Webra Speed 40 on a tuned pipe. I didn't scale it up at all and it goes great. I did however build it too heavy.
I really like [link=http://www.snhflyingeagles.org/pdf_files/GremlinConstructionPhotos.pdf]these (click here)[/link] alternative construction instructions and on page 7 it shows how to taper the end of the ailerons to avoid high speed flutter with the bigger engine. On mine to further avoid flutter, I also did not taper the aileron towards its trailing edge (used flat balsa sheet), I added many more hinges than stock and I pinned all of the hinges. With these mods, I get no flutter at any speed including wide open straight down. My mistake was to use bidirectional strapping tape over the entire wing. That stuff is heavy. I think if I was to do it again I'd use the standard construction methods (plus the anti-flutter mod's) but maybe just push the glassfiber bandage center out an extra inch or two.

All the best

Barry