GXP's have bilsteins already so the struts can handle a lowering spring. They also already sit a little lower than a regular 04+ grand prix. I do know that SSC makes lowering springs that seem to perform well for the GXP. There are not a lot of mods available for the GXP. I am pretty sure that there are no coilover set ups. Eibach also makes springs. I'm not sure how they ride or perform.