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As the above poster mentioned, the HP and TQ increases are the result of the true dual exhaust (30% less back pressure) and tweaked ECU. Coming from a G37S I can attest to the fact that there isn't an appreciable difference in power; however, I do find that the engine winds-up a little faster over 6000rpm and the sound is less thrashy as compared to my S.
Not sure, but I'd be willing to bet that the cost of doing that might be the same as an aftermarket or possibly even more. The Motordyne has a slight power increase and sounds amazing...thats what I would get. But of course that all depends on what your goal is I suppose.