Dick,
An over-all good advise you gave here.
In a MCP or "tub" exhaust, more volume is equivalent to less sensitivvity to setup accuracy over the whole engine's rpm range, unless you want to tune the volume to a specific rpm response. :_) (I love it when I depart on simplicity). With a certain volume on a specific engine, a gain will be noticed over open exhaust measurements. Sometimes the engine porting tune is such, that you <must> use some kind of a tuned system behind it.
This however should not be the case in airplane engines since they drive quite a different load than road racing engines.

Winfried,
In general, if given a choice between the 80cc or a 50cc kanister muffler, go for the larger one, if you can manage it in your model.