to be honest it's more of a sound game now a days then performance unless you're feeding your car enough air to need the extra flow. i'm partial to dual setups though just for the sound alone. hit up youtube and just listen to as many clips as you can until you find the right setup for you. as for an exhaust netting you 15 hp...well maybe with a tune, an o/r midpipe and full exhaust.
carving corners like a boss