Now that you have all this air flowing into the system, you have to get it out the rear of the car just as well. A low restriction exhaust system will allow the exhaust gasses to flow with less restriction. This will help to reduce exhaust system pressures allowing greater boost from your turbo, reduced lag, and less stress on the turbo itself. Things to look for in a cat-back system are: mandrel bending (vs. press bending), as few bends as possible, bends which aren't sharp, some sort of corrosion protection (stainless steel, and aluminized systems), and SIZE. Yes size does matter guys :).
A list member mentioned that if you are planning on getting the most horsepower that you can out of your DSM you might want to spend the money and go with a 3" mandrel bent cat-back system to begin with. This will help you to eliminate having to replace the system twice when you get so many mods on there that the normal low restriction system gets too restrictive.
If, however, you aren't looking to go past the 12.5 second mark than you could easily get away with only a 2.5" system. The price is considerably less than the 3" system and the noise levels are tamer. From what I've read on the digest the gains from the larger 3" exhaust are minimal (for the price) until you start doing things like getting new turbos, porting the head, and getting into boost levels which require race fuel. The money you save on the 2.5" cat-back can then be spent to buy the mods which will take advantage of your new breathing ability.
The latest information is from Turbo Magazine (1). They did a very nice test using a dynamometer to evaluate the performance benefits from a Thermal R&D 3" mandrel bent stainless steel cat-back system. The two important conclusions from this article are that 1) Peak power increased by 5.8hp at a LOWER RPM and 2) The average power gain from 2500-7000 RPM was 5.5 HP. Think about that. Not only are you getting more peak HP (which is marginally important) but you are getting it in the entire RPM band. In fact, the largest power increase was 8.3 and it occurred at 4000 RPM. Can you say mid-range guts!!! Here are the two VERY informative charts from the article. Chart 1 and Chart 2. Remember these are wheel horsepower so they will be less than the factory claimed flywheel horsepower. What the article doesn't show however is the improvements in turbo response which everyone, and I mean everyone, on the digest swears by. So don't think that the small numbers mean you won't feel it. I drove in another member's 97 GSX with a 3" cat-back with the boost set around 17psi and let me tell you . . . it rocked my 97 Talon AWD with stock exhaust running 15psi. It wasn't the power that I noticed, but the violence with which it slammed me in the back. I wish there was a way to quantify, easily, the changes in turbo response. I think an accelerometer would show the increased rate at which velocity increased compared to a stock system. Anyone have a data logger capable of taking this kind of data??? And let's not forget this test was done with stock boost. The gains from adding a freer exhaust will be much greater once you are flowing more air with raised boost levels.
Also interesting to note is that compared to an open cat-back exhaust the 3" showed very little difference and at some points in the dyno chart the 3" is actually BETTER than an open exhaust. Over all looking at the average HP increase over the RPM range the open exhaust was only .49 HP better than the 3" exhaust (5.99 vs. 5.55).
(1) Information courtesy of Turbo and High Performance magazine Huntington Beach, CA July 1997