I have a '97 Mach 1, and when the engine was stock, I dynoed a DNE 3 into 1, which I gained about 4 h.p. at the track from 50mph and up but lost about 2-3 h.p. from engagement to 45mph on the track dyno. I also track dynoed a Fast 3 into 3 (3 individual silencers), and gained about 2 h.p. from engagement to 40mph, then lost power, up to 10 h.p. from 50 - 80 mph. I now have it trail ported, shaved head, V-force II's, and engine dynoed 6 more h.p. and 4 ft.-lbs. of torque with a MBRP 3 into 1 muffler as compared to my stock muffler.
Hope this helps.