I wouldn't necessarly count on the OS being that much more powerful. A 10.5x7 isn't a common prop in the US, so I doubt you'll find actual tach readings on that engine to compare to. Your best bet is to find someone local and tach their engine.

Also, keep in mind that where you are makes a big difference. You loose a lot of RPM as you go up over sea level. Air temp and humidity play a part there as well.

Oh, and off the cuff, for around here, 12.8k on a 10.5x7 sounds pretty good to me, with a 10x7, I'd expect that to climb to 14k, which isn't out of line with the OS by much.

Oh, sometimes just changing mufflers can pick up RPM as well. The TT stock muffler or the Tower muffler both will increase the RPM on an OS over the stock OS muffler.