Wider tires on 2011 GT500 to compete with the upcoming ZL1

So the new ZL1 comes on 305 rears, whilst our 2011 GT500s come with 285s plus a reputation for not being able to hook up.

I've done some research over the net, and people have successfully installed 305s on 10" wide wheels (I have the SVT package). In fact many wheels recommend 9-10" for a 305.

The question now is: which is a good street tire that's 305/XR20 (whatever the X is supposed to be to be same diameter as the stock wheel)? And what does "X" equal?

Thanks!!