I'm about to add a glasspack to mine to eliminate the crackles on it. If you do this though, make sure they put it in backwards. By doing this it won't change the tone or slow down the gasses. It will just reduc or eliminate the crackling and quiet it down a bit.
I was thinking about adding a straight-thru perforated core glasspack to my custom exhaust. My question is, if you buy a straight-thru resonator/glasspack, what difference will it make if you mount it frontwards or backwards? It's still straight-thru regardless of how you mount. I don't know, maybe I'm about to learn something here.