I have not had any experience with glasspacks, but they are pretty much a straight through design. I believe most will still in time burn out the packing and get even louder. I believe if I were to do any type of glass packs I would use two individual ones since they are basically like a piece of inline pipe. You could just have a y installed to go to both like your dual outlet setup or install the x-pipe. X-pipe is suppose to give you a little more torque. I know Pypes makes a muffler called the M80 that has steel mesh packing, it might last longer. It is kinda like a glass pack and not expensive. Those sound good on the mustangs, they are recommended to be Mid-mounted not right at the very back. Remember this, the shorter the glasspack the louder it will be for sure. I wish I could help you more, just research your options before you decide to do anything.

 

Cherrybomb glasspack. Single 2.5" in/out

 

Glass packs are just casings, essentially. The pipe running through is perforated, and the space between the casing and perforated pipe is filled with fiberglass. Shorter = louder, AND less back pressure. Which means more power. (Probably not noticable unless you have a lot of backpressure...)

 

Alright thanks I'll check em out

 

Have decided on the SI/DO Glasspack Extreme muffler

 

I believe you mean the chery bomb extreme. It is a case muffler,not a glasspack. But is made by the famous makers of the original Glasspacks. The extreme is the one I was thinking of and not the vortex. The extreme does sound good. It is short like 4"x9/34". If you want loud you made the right choice. I actually thought about it but thought it might be a little too much for me. When you get it installed if possible post a short clip. I do like the sound, very throaty and good.
here is a clip of a 2009 SI/DO, Just double click the link.
http://youtu.be/T95rZF39Z3g

 

Yeah that's what I meant haha, it's whatever, they all sound better than crappy stock