You'll gain like 5-7hp from the borla. Everyone around here seems to love the Borla sound, but i've personally heard it in person and with a header, original cat in place, and an added resonator it definitley still sounded like a burner(it had an acute, piercing sound to it). But if you don't mind it too much, go for it.
I always got compliments with mine when I had the Cosworth race header on, even though it was pretty loud. Now, with the turbo, it sounds amazing! I love how it sounds, and loved how it sounded before I had the header too. I have the Borla flex-back as well. [thumb]