I will say this: the exhaust coming out the tailpipe is much much cooler than the exhaust in the headers. Probably on the order of 1000F-1500F cooler. Not that 400F at the tailpipe is a cool breeze, but Cody is right about the exhaust cooling dramatically.
As far as the pv=nrt argument, I actually remember that thread on I-club.
What a nightmare of people making up science that was.
More to the point of the thread. I went from that Invidia to a catted turbo xs DP and prodrive muffler. It's not much quieter. Sounds a little less angry, but if you're really looking for something stealthy, figure out how to bolt a stock sti muffler on there or something.