Anything you take out..I.E.-muffler, resonator, cat, etc..will make it less restricted, thus giving little gains. But replacing with a glass-pack won't give you as much as if you removed the muffler completely and left it off. Also, isnt the glass-pack just like a resonator and not the muffler at the end? So, that would mean you replaced your resonator. If that's correct, then you really don't need the glass-pack or any resonator since all it does is reduce the sound.