Asmund,

I do have a few with fairly rough faces on muffler or exhaust stack... Not criticizing your point, just adding my experiences.

I use brown gasket paper about .25mm (0.01"?) thick, and retighten the muffler bolts after the first few runs. Once it has had heat, vibration and oils exposure, the paper lasts very well. A hobby knife tracing the inner and outer outlines works quickly and well.

Why use gaskets? I don't like oil weeping from the joint surface, as it often does even with very nicely (apparently) finished mating faces... And, if the bolts loosen during a flight, the metals can beat on each other, darkening the exhaust oil. A gasket prevents that.