I looked up some rear rotors for a 2005 Cobalt SS and compared the part number to some for a 2010 Cobalt SS. The part numbers are different, which means that they might not be interchangeable. I haven't found any definite answers yet though.
EDIT: Through some more searching, I found that the SS 2005-2007 rear rotors are 10.079" in diameter. The rear rotors for a 2008-2010 SS are 11.5" in diameter. If this information is correct, then in order to fit the LNF rear rotors on an LSJ, the rear caliper would need to be replaced with a larger one. You're probably better off buying an aftermarket rotor made for the LSJ.
Believe me, these cars have a habit of growing on their owners. Your feelings will only strengthen over time...
Last edited by Parthros; 04-29-2016 at 06:43 AM.