a muffler mod, means cutting a small hole in the muffler to let the engine breathe better, which on most modern saws, is a must do, plus richening the carb to allow the saw to run right. I mod just about every muffler on every saw I get, unless it was built in the 80s. those generally dont need to be modified.
the 8 t rim is a larger sprocket, which gives a 14% faster speed on the sawchain, which on short bars, like a 18" or less, will give good speed in almost all wood. sorry I forgot to explain this before. hope it makes sense now.