With the stock muffler, my O.S. 50 Sx was turning an APC 12.25X3.75 @ 12,400 rpm
With a Tower Hobbies muffler same prop. 13,200 rpm.
I took the muffler into pieces and I made a longer tube for a total capacity of 200 cc., the same capacity of my Macs tuned pipe including header and silicone hose.
Now my engine with same propeller is turning at 13,800 rpm.
The original tube for the tower muffler is 90 mm long for a total volume capacity of 150 cc, mine is 120mm long.
Hope this will be usefull for somebody.

Regards.