are you looking for sound over performance? do you like really loud exhausts?
if you want performance your better off going with AA or UUC
if you want loud go with Tri Flow
if you want loaud and performance get eisemann race
AA - louder than stock M3 , very nice tone, top performance(my personal favorite)
UUC system U- drones at high speeds, pretty loud, nice race sound
UUC TSE - not really that quiet. , good hp gains, most like it
Eisenmann Race - very very load, drone like crazy, $$$, performance is good,
tri fow, loud at low rpms. drones, doesnt sound as good as the rest.
thats just the main ones out there that i have herd personally
btw im selling an AA gen 3, its 6 months old.