I don't know what you are doing, but my Mag .28XL with KY mousse can is turning over 18k with the MAS 9x4 on 15% Omega fuel. (I was tached at 18,400 at one contest, but I don't normally use a tach, mine is broken).

You should turn over 15k with just the stock muffler, never mind the can. I'd say the engine isn't broken in yet, or you're having some kind of fuel or carb problem. Or your tach isn't working right. It's possible that the can is causing too much back pressure. How does it tach with the stock muffler? Also, in theory, you want the inner pipe to be "tunned" to specific length and distance from the back wall of the can. With the KY cans, I can sometimes increase RPM by moving the can in and out on the silicon coupler, though I usually don't bother.