Very interesting reading. It supports some of the opinions made in the forums that continually get shot down. The concept of keeping the pipe size small enough to keep the density of the exhaust gas uniform enough to keep from adding back pressure due to cooling is hard for many to grasp.
Also, the header issue. On the Turbo, the exhaust manifold and turbo are serviced as an asembly. The Service Manual cautions not to remove the turbocharger from the exhaust manifold, as exhaust leaks will result. The Service Manual recommends that the turbocharger elbow be replaced along with the turbocharger/exhaust manifold assembly.
This is not to say that it couldn't be serviced by someone competent in exhaust systems. However, it does tend to remove the aspect of replacing the exhaust manifold with a header.
The faq at the link above also points out the low end torque sacrifice found when going to a header or larger diameter pipe. The quick spool and low-end response are among the larger advantages of the GT as it comes stock. Eliminating the low end torque in exchange for some near redline benefit will likely limit the fun of driving the GT quite a bit.
I don't know what the final solution should be. The sound is vastly important to many. I would like for mine to be a little more assertive in the exhaust sound department. I am leaning more toward a resonator and no muffler; if I can find a resonator of the same diameter of the stock piping. If so, the muffler could be removed and a resonator or straight pipe put in it's place. With the alteration being so close to the actual exhaust outlet, it is likely that it will not alter the exhaust gas flow.
Getting the muffler replaced with a straight pipe, or a resonator, and having a chrome tip added (the local shop offered to cut the one off the muffler and put it on the end of the pipe)is probably around $400.00 cheaper than ordering an aftermarket setup, paying freight and having it installed.
I am still in the "data collecting" stage. All in All, the stock system isn't as bad as many think; when it comes to doing the job properly to assure the best power curve. Taking advantage of the natural tendacy of the Turbo to quiet the sound before it gets to the muffler, simply eliminating the muffler may be the best approach..