No kidding... it works.

The GMS/TH engines seem to develope enough of a vacuum that even with no pressure from the muffler straight up flight is not overly problematic.

When my muffler fell off I was doing aerobatics at a good distance from my position, so I did not hear the change in engine loudness.

I was performing straight up flight, inverted, etc. at a good height. I noticed almost no change in the flight characteristics.

Only when I brought it much closer in did I notice the rather loud sounds emanating from the front remaining muffler half.

I ended up making several more flights this way before leaving for the day.

I did note that the amount of leeway I had in the tuning dropped greatly, but it still worked.