Sorry, I missed your last question. The horsepower gain is not noticeable if any. If your car has the free flow cats they don't restrict that much anyway. The sound is more crisp not louder with the straight pipes.
Now you have the right idea. Don't take the whole car they may have a problem with that. Actually if you take the length of the cat and do a trace of the manifold ends on paper they could make them up from there.
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