The OS FX, Irvine, Norvel, and Webra engines are all noticeably more powerful than the GMS, TT, and OS LA engines. If everyone else has a GMS, you'll be fine, but if anyone has a more powerful engine, you'll find out that the difference is actually pretty large.

The Magnum .28 acutally isn't bad on power and can be had with out a lot of money. I still run 3 of these in my B class planes, on mousse cans. The Mag .25 is heavy and underpowered though (in the same class as the GMS for example), don't buy it. The Mag .28 needs carb work to stay reliable, and isn't legal in the various scale combat classes, so if you want to try a scale class later, it's not a good choice.

By "Macs Muffler", do you mean the curved black single part one? That's actually not a tuned muffler, and doesn't improve performance very much over a stock muffler. It's nothing like what you get with a mousse can or Ultrathrust.

Anyway, the best beginner's combat engine is still the OS FX, hands down. Solid power and very reliable. If you are good with engines, there are other options, but if you don't think of yourself as an "engine guy" get the OS.