Downside: You are still stuck on the HP thing... just let it go. Been there done that. When I first started RC I made spreadsheets with engine HP, weights, costs etc.... waste of time. It meant nothing. Some engine makers, like YS, don't even publish HP ratings, but we all know YS engines make monster power.

As for Webra and Rossi... I really don't know much about them. I do know many Rossi engines are used in pylon. Set up properly with a small diameter high pitch prop I know they scream. But put a sport prop on and who knows. Probably lask luster performance like what Ed just posted.

I just looked up your plane on the web... seagull edge 540, 60 sized. ~9lbs finished. The OS91FX you have in there now should pull a 9lb plane just fine. It won't have unlimited, but it should be close to 1:1 if proped properly. Are you sure your engine is working properly? Plane weight? What prop are you using?