I won't weigh in with an guess of how much power can be gained from exhaust work, though I would tend to believe it was less then the 20 HP mark, as it is taking a new PCM and injectors to get that from Mopar.
The design that is used on the turbo is tuned for the turbo engine. With the amoung of noise that the turbo breaks ups / absorbs, straight pipes become a possibility. At a minimum, a resonator the is more free flowing would be a thought. It would be nice if someone could give a measurement of backpressure measured with the stock muffler, and various other brands of mufflers, resonators, etc. when fed from the same diameter pipe as the stock muffler is fed from on a turbo charged engine.
However, I will point out that the 2003 Service Manual defines the TurboCharger as an exhaust driven supercharger. Other than the HP lost due to the Super charger being belt driven, they both apparently provide the same outcome.
Although, I would tend to believe that the turbo charger's performance would vary more with changes in the exhaust system. Any change that slows the extraction from the tail pipe would have the potential of limiting the speed of the turbo. One example of changing the extraction might be enlarging the exhaust pipe to the point that the exhaust gas temp lowers from expansion, becomes more dense and slows extraction.
Since the GT uses what Chrysler considers and assembly for the Turbo Charger, wastegate and exhaust manifold (any one of these items become suspect, and the diagnostic manual says to change the whole assembly), installing a header would not become an option. From the physics end of the discussions, increasing the size of the exhaust flow before the cat (like going with a header) would actually decrease low end torque, so that is one worry we don't have to contend with.
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