I don't think Kooks does anything cat back for the '04s. Get the LTs and keep the stock exhaust. You should be very pleased.
If you go to YouTube and punch in "2004 GTO exhaust" and variations similar to that you should find plenty of clips.
Current: MGW-p, H-pipe, SS OTRCAI, Lovell rear springs, Eliminator shocks and Pioneer AVH-P3300BT, JBA LTs+catless mids, Drilled/Slotted rotors, Kooks Afterburners &Catch can.... so far. Next: Tune Wish list: Intake & TB, modest cam, UDP, gears