There is a power increase when the cats are gutted, I'm not sure how much as mine hasn't been dynoed, but I would reckon on around 5-8bhp with a cold air induction kit fitted as well, the major problem is the manifold itself, it's very restrictive, I've heard that a full Janspeed gives around an extra 20bhp so that gives you an idea of how restrictive the original manifold is, the sound from a gutted cat is a little louder and a bit 'raspy' if you know what I mean, although in my opinion it's not an unpleasant sound
