I've been looking into the TSW Nurburgrings and the Forgestar F14s to replace my steel heritage wheels, but if you're doing autocross I would imagine your best bet is to do with wheels smaller than 20".
Even Pedders went with 19's on their build:
http://www.camaro5.com/forums/showthread.php?t=90270
Anyone have solid numbers on how much the 18" Steel Heritage Wheels on the LS weigh? 18" TSW's and Forgestars are about 18.5 lbs . . . which is crazy light.