If you take the end of your exhaust off and look down into it, you will see one large hole and 2 small holes way down inside. I believe the small ones are the main source of restriction, so those should be drilled larger. I will do a few simple math equations tonight and figure out what size you want those to be.
The large hole is what the tube inserts into. Some have cut this tube shorter, essentially bypassing those 2 holes. That will work as well but will probably be louder than by drilling the 2 holes larger.
BTW, this is all theory, I havent actually done this, but I think it is a sound idea. But, these should de-restrict the muffler and still keep it sounding good.
Again, I wouldnt do too much to the exhaust unless I was running a larger carb. On 2 strokes you can easily have too little back pressure and get worse performance.