I tested the AX with the stock, Power Box muffler and with the Tower muffler. On a 3D, 12.25-3.75 prop, I gained 800 rpm. Tower mufflers are very popular on OS & other .46 engines.

In the photos, the large round muffler on the left and in front is the Tower. Next to it with the extension in the center is the muffler for a Magnum .52XLS. Third is the GMS .47 Standard muffler, as opposed to the "tuned" muffler, which is the same as the Tower. Both engines are made by the same manufacturer. The last muffler is the OS P-Box. The last photo shows the Tower next to the OS. The Tower is the largest in volume by a good bit. It's not that much longer, but the diameter is larger. It also has no baffle.

As for breaking it in, I would start with the Tower, if you plan to use it regularly.