Gene,

The Mac's one-piece muffler's only advantage over the stock silencer, that came with the engine, is its weight (and size).

It is not "tuned" in any way, nor does it have any added expansion volume, or a large outlet, that may potentially boost the engine's power.
To make matters worse, it now carries a prohibitively high purchase cost, that only American modelers (and wishful believers elsewhere) are willing to shell out, given its very few advantages. As if $38-$43 is peanuts for anyone.

If you want some added power, the Tower/Hobbico .46 muffler can do it for you, and if you want even more power, the very expensive Jettstream, or Ultrathrust, or other true tuned exhaust systems.

If you want more power for nothing, drill out the exit hole size of your muffler, or for a few bucks, extend the expansion chamber of your stock silencer. It will help more and will not cost you nearly as much, as the Mac's one-piece.