The muffler, whilst fine for scale applications, is extremely small and will respond to anything that increases or decreases the outflow of gasses. Sealing the seams will reduce power. Adding an extra stinger pipe will increase power, at the cost of added noise levels.
It is not the purpose of this question (I hope) to provice a course in soldering or brazing ??
If serious power is needed, get rid of the muffler, and fit either a MVVS 3271 or a MVVS tuned pipe, or canisters as provided by several dedicated suppliers.