Your tires should last at least 20K if you drive it decent. Mine isn't lowered either and noticed the inside of the rears wearing more than normal. On most Crossfires you can see the negative camber just following them.

There is no adjustment on the rear camber so the alignment shop will just let it go as is. I installed the adjustable camber arms and now my wheels are pretty close to 0 as that should curb the inside wear on the rear tires.

James