You can use Game Stock Car mods in AMS, they work pretty well, although doesn't use AMS enhancements (like flat spots). One of the best is BES 2012 (Blancpain Endurance Series 2012), created by EEC for GTR2 and converted to GSC. It is a very light mod (run smoothly on lower rigs), has several GT3 cars from last generation (Ferrari 458, McLaren MP4-12, BMW Z4, Audi R8 RMS Ultra, Lambo Gallardo, NIssan GT-R, Aston Martin, Mercedes SLS AMG, etc), and all skins from 2012 season. You can find it on Xtreme Factor.
Aside from that, the current EEC GT3 mod is the way to go. Those cars have really haevy graphics, so a strong card is needed. And, since the base mod have only the cars (not the liveries), don't forget to get all the skinpacks for it here in RaceDepartment. Each series has its own set of cars, recreating real-life championships (for instance, you can find the Ginetta only in British GT series, or the Cadillac CVS only in Pirelli World Challenge, and so on).
If you want only Reiza-quality cars, I would suggest to step down a bit, and using the Boxer Cup with the Porsche Super Cup skins. They are Porsche GT3 Cup, so you can quench your craving for GT3 that way
Last but not least, don't forget that Stock Car Brasil handles even better than GT3 cars in corners, so give them a chance too ;-)