Moby,
I have a mongoose on My E4 which I bought secind hand ,taken off an E5,try second hand market 1st as its stainless it doesn't rot.should get a deal on a turbo back system.
The sound isn't to obtrusive in the car ,when its hard acceleration then the cops can hear it ,to my dis advantage a while back 70-120 cops gave chase ,just because they heard the hardness of the exhaust bellow.
Alot of others exhausts could drive you nuts on incar noise especially on a long journey.stiil will give a reasonable gain in hp with a decat pipe.I like a loud exhaust on other cars but not too boomy on my own.