3" vs 2.5" exhaust systems

How much H.P. can I expect to lose going for a 3" t0 2.5" exhaust system? I am really thinking of going to a 2.5" system in order to move the system under the car between the frame rails and out to the rear axle to quite it down. I don't know what to do running fenderwell headers and 3" exhaust out the side before the rear tires, it's just to loud for me on the street.