I read in a Camaro performance handbook that wider tires actually make handling worse instead of improve it. Well this all depends on how wide you want to go of course and what you would be using it for....if it is just for looks then you should have no problem. As for how wide you can go, I do not know. Have fun.
I have to agree with Meat. the 235/60/15 is the best for the stock rim. That is what I have on my 89 RS. The stock size is 215/65/15 so the 235/60/15 is a little better. Those will make you car handle better. If you are looking for traction at the track, just drop the air pressure in the rear tires to 10lbs.