there are a few more things that need to be done at these rpm.
shorten the header
check the carb intake manifold. It should allow the air from the larger carb to pass
smooth the crankshaft inner gas passage.
set ignition slightly later. (undo one screw, and rotate the sensor

After these mods, you will have a few 100 rpm more.
The modified muffler will be a lot louder. Better is to reduce the inner flow resistance, which is quite high, and leave the muffler on. It then tends to act as a low pass filter, and make the engine sound much more pleasant than without the muffler.