Congrats.
I had a scorpion years ago on my gti-r, was the best sounding exhaust IMO. It wasn't too loud, but sounded good when you heard the mrs. driving off it for instance.
It was in the same back box design as the standard exhaust system. Tail pipe was nice too.
Only problem is, since having a mongoose I think you'll notice the power difference. The mongoose sounds terrible IMO when just trundling along and I feel like a right plonker sometimes, but on boost it sounds like it means business and seems to give some decent power.
If I was you I'd look into getting a system made up. Use the standard pipe run, but instead of having the pipe go into and out of the backbox the same end, make it go in one end and out the other. So you'd be able to have a rear silencer of reasonable size parallel with the rear bumper.
You'd need to block the original exhaust hole and make another the other side, but shouldn't be too hard.