I am not comparing with pitts mufflers. They are exhaust gas collectors, not mufflers, and only serve to harm the engine's potential.
the difference between your OS 108 and OS 1.60 should be larger than a mere 10%, and yes, the gas engine will pull about 1 : 1.5 lbs less than the comparable glow engine of same displacement. I have measured 19 lbs pull on my 26cc test engine, using the MVVS tuned pipe.
That is more than enough to adequately hover a 12 lbs plane.