Do performance mufflers really make much of a difference in performance? I have heard that certain aftermarket mufflers actually cut down on hp. Please help me decide whether or not to get one and which ones might be good to get.

MOST mufflers reduce power when compared to a straight pipe (I only know of one that made power and I can't tell >>). Most aftermarket performance mufflers will increase power over stock when combined with a well designed cat back. Just adding a muffler to a stock system will not do anything other than change the sound.