Hey, I cut out the OE cat from my 99 SLE. I had the code for the rear O2 sensor disabled. It really gave it better performance, and I think better gas mileage. It'* been said that a high flow cat provides the same result as just not having one. I believe it since my friend 1337ssei actually went from a high flow cat to no cat twice - with both of his L67 cars.
Going from an OE to no cat or a high flow is excellent. You can pick up a high flow Magnaflow cat for about $60 shipped on eBay. I sold one at NEBF for $40. My SLE now has an OE cat again due to some people being poopheads, but I assure you it was for their profit and with no law in mind. If I had anything to do with it, I'd still have no cat or at least a high flow.
94 STEi and 99 SLO