Here is a bone stock 05 GTO. Put down good #'s…got to love the Tq of the LS2 such a flat curve. I’d say the 400 hp is pretty accurate, it lost 58 HP to the wheels and 58/400 is a 14.5% loss to through the drivetrain.
GM richens the tunes up a bit… but if you lean em out just a tad, you should see 350+