I installed my Greddy 4-2-1 SS header a couple months ago. Seems like a mod that needed to be done.

Here were the differences I noticed:
- sound - didn't really notice a difference; CAI, catback, ES100s, and mp3 player generate a lot of noise
- butt dyno - you can convince your butt dyno of anything. Did the header make my car go faster? I guess. But then, so did my spoiler!
- looks - I like the look of the header in the engine bay; much cleaner/better than that stock heat shield (which blocked the stock header monstrosity)
- weight - that 2-piece stock header monstrosity sure weighed a lot

Am I glad I did it? You bet. :thumbup:

Have fun installing it!