Ive done something like what jezusjonz did to his muffler but my exhaust pipe was different. Mine has a thin tube connected to the tip of the exhaust that forces the gasses to travel much about three times the length of the muffler, i bought a longer screw for the exhaust tip and put a spring between the tip and the baffles. Now a brake line connected to a lever holds the muffler closed and when i get away from residential areas i can hit the lever and go.
Anyone who says that removing the exhaust just makes noise has obviously never tried it. I was very impressed with the power and quick response of the engine with the muffler disengaged.