A 12.25-3.75 should be just fine. I have run that size on mine with no problem. It will not over load or over heat your engine unless there is some problem with your specific engine. I have a GMS, Tower, OS, Magnum, Evo, Thunder Tiger, all in the .46-.52 size and they all run fine on that prop. I use that prop, an APC 12-4, APC 11.5-4 or an 11-6 on all these engines with either 10% or 15% Omega fuel.

The fatter muffler with the shiny center is the "tuned" muffler, usually known as the Tower muffler. They will normally add 700-800 rpm to OS and other engines. The muffler with the fins is the "Standard" muffler. I run one of those because I did not see much difference in rpm on that engine with the tuned muffler, maybe 200 rpm, while the OS & Magnum gained 700-800 over their standard mufflers. The prop I used for the test was a 12.25-3.75 APC.

The tuned muffler has a larger volume than a standard muffler and let the exhaust flow more freely. It does produce less pressure for the tank so you have to tune a little richer for acro planes or expect them to lean out in tight maneuvers. As I recall, the tuning was different from the tuned muffler to the standard when I made my rpm tests, but that is to be expected with tuned exhausts.