"you really need LT headers"... and a tune...
1 7/8" headers.
I had cats... deleted 'em in favor of cutouts... if I hadn't done cutouts, I would have kept the cats.
I think a 2.5" exhaust is plenty unless you are planning to go above about 800 rwhp.
I honestly think LT headers are a MUST with these V8s (unless you're going with a twin turbo set up later). They are probably the biggest bang for the buck hp gain you'll get.
You could wait on the exhaust if you want to be "thrifty." The headers do add a lot of sound on their own. Exhaust pretty much is only a sound thing. However, I HATED the looks of those awful stock garbage cans, so I got rid of them and said hello to Mr. Borla.