If you're looking to maximize performance (i.e. flow) and minimize noise, go with anything that is a perforated core center tube with an offset inlet versus outlet.
This will give you the best of both worlds.
Walker Dynomax UltraFLO, Magnaflow, and Borla all makes mufflers like these.
Many are stainless steel which can handle the high heat of a rotary's exhaust.
Try and find mufflers that use "stainless steel wool" as packing, as this greatly extends life over fiberglass packing.
As a rule, bigger the muffler, the better the noise suppression.
-Ted
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