with a good thourough tune GXP will net 25-28mpg hwy without DOD, most people havent gone to far into the 5.3 realm of things and the 3.8 is all they know. I had a GTP and there a **** ton of engine mods you can do. most any LS part will interchange. it all comes down to what youre wanting. and to the person who said our headers were 1000 bucks youre out of youre mind.