the air lid set-ups add a bit. I don't know much about the SSRA set ups but my SLP lid made a noticeable difference. muffler didn't really make any difference other than noise, then other than nitrous my next mod was an Ls2, cam, stall, trans, etc.
They LOVE nitrous though. won't add to the top speed at all but it will get you there a hell of a lot faster.
