1998 was the year of the GS DSM, with our 97 Talon winning more ProSolo events than any other car in class, and the 95 Eclipse of Mark Allen nipping us out for the Pro Finale and National Championship.
Since then, the SCCA has reclassed the DSMs into DS
Here's the setup we used:
Wheels / Tires : We used 245 45R17 BFGoodrich R1s, on stock 97 GSX wheels with 1/4" spacers. (they're 2lbs lighter than the Talon wheels)
We had to shave the tires; unshaved R1s would get the Evil Groove of Doom and cord early - although now that the R1 has been replaced with the G Force (that are molded with less tread) that's probably no longer true. 2003 Tire shaving is a thing of the past, the top-flite Kumhos and Hoosiers are ready to go right off the rack.
Now that the BFG R1 and the G Force are no longer in production, the tire to have is the Hoosier. 245 40R17 works.
While you are allowed aftermarket wheels, there are no 17" X 6.5" aftermarket wheels. Period. They don't exist. Cram as much tire onto the wheel as you can, and hope for the best. Note too that the Eclipse wheels are marginally lighter than the Talon wheels.
Alignment : There is no way to change camber on a stock 2G. We tried the "loosen all the bolts,
pull on the wheel, and retighten" trick, and the camber came back to the tenth of a degree!
We ran 1/4" toe out up front, and 1/8" toe out in the rear, and with this setup we could change the handling from tight to loose with a 2lb air pressure change front to rear (34 PSI up front, 31 rear) However, this setup bit us on the 98 Nationals North Course, where the car was too loose at higher speeds and air pressure wasn't enough to balance the car.
Note that while this setup works great for competition, highway driving in incliment weather is not an enjoyable experience. The car is twitchy and unstable, especially in slush, and not much fun to drive.
Shocks : We ran (and won on!) stock shocks all season. The car rolled like an old canoe, but hey, it worked for us - with one possible exception, the Nationals North course. Our opinion is that adjustable shocks would probably have allowed us to tighten the car up where air pressure didn't. Examining pictures of the Mark Allen car (which had Koni double-adjustables) and ours taken at the same point in the course revealed much less roll than ours.
A lot has changed since those days. For one, we've learned a lot about how the car works. Secondly, the price of effective aftermarket shocks has come way, way down. And thirdly, the current tire to have, the Hoosier, is much more camber-sensitve than the BFG R1 was.
The major issue you fight on a Stock DSM is that the outside front tire loses camber (with respect to the ground) faster than it gains (negative) camber as the outside front suspension compresses as the sprung mass rolls onto it. This means that the more you roll, the less tire is in contact with the ground.
To keep that from happening, you want to limit the amount of roll. Because you are prohibited from changing springs, but are allowed any shocks you want, running super-stiff shocks can slow down the roll rate to the point where the tire makes better contact with the ground.
Koni Sport shocks (aka Koni Yellows) work well. Pretty much any other shocks are garbage.
Note that this suggests that a really big front anti-roll bar (which is allowed in Stock) might work very well. We never tried it ourselves, but it is something that I would investigate were I running a Stock car.
Engine : We used a K&N stock replacement filter, and a Buschur Racing cat-back 2.5" exhaust. We don't have any hard power gain numbers, but the exhaust dropped a huge amount of weight! The stock muffler is enormous!