x and h pipes

An Ultra Flo or Magnflow or any other number of similar mufflers are nothing more than a glass pak with more packing material. Certainly the sound can be modified but 2.5" pipe is going to flow the same regardless if it has a small case (glass pak) around it or a much larger one (ultra flo / magnaflow).

That being said the raw flow rate capability of a muffler is of little importance. They all flow more than the engine can produce providing they are the proper size for the engine.

Not long ago I read a SAE article on designing a zero loss exhaust system and the simple take away was that its simpler to design a zero loss system with a chambered muffler than with an absorption muffler (glass pak / Magnflow). The reason being is the tail pipe after the muffler become irrelevant in the overall design where the tail pipe looks like a restriction to the system with the absorption muffler and needs to be factored into the design.