[Msquared]

I'm not clear: you have an SS without the 1LE package and you're currently running ZL1 (non-1LE) wheels on it, which are 20x10/11? While there's nothing mechanically wrong with that, it puts you in very different autocross classes, depending on which events you're planning to attend. In SCCA, it would move you from F Street to either CAM-C or ESP, which is a very different level of car mods overall. So you need to decide what class you want to run first.

Regardless of that decision, I think the Falken RT660 is the way to go on a Camaro right now, with the Nankang CR1 maybe as a good alternative (reviews and results are all over the map with that tire from what I see). The Yokohamas are very fast, but they won't be happy on a stock or near-stock Camaro, and you'll need heroic measures to keep them cool enough to be happy at an autocross with 10 runs. Everything else that's streetable is slower. I think you'll be able to do 100 runs plus a little street driving on the Falkens before you have to replace them.

If you want to stay in F Street, you need to stay with stock widths and can go down 1" in diameter, so look at 19x8.5/9.5. Then look at the Falken RT660 in 275/35/19 for the rears and either 245/35/19 or 265/35/19 for the fronts (not sure which is the best bet - they don't make a 245/40/19, which would be ideal).

If you don't care about staying in F Street, then go with CAM-C. ESP would be a bad choice, since the only good tire for that class is the Hoosier A7 and that is definitely not streetable. In CAM-C you have to stay with 200-treadwear ratings, but you can run any size you want. An easy choice would be 19x10.5 front and rear with 305/30/19 Falkens or Nankangs. I have a friend running 19x11 on his SS, which would be even better. I'm not sure if he's running camber plates or not, but they don't rub. But in CAM-C, you're going to want camber plates and several other parts to really start improving the car's potential...which then becomes a money pit...which is why I'm not going there any time soon! So consider that carefully.