I'm just a V6'er so I hope you don't mind me chiming in. I can't speak for changing out the rear end gear, but I have to disagree with Pentalab about the tune. While he is right about the hp gain not being much to write home about, the 5R55S automatic we have does benefit form a hand held tuner. If you get one that is specific to your model and year, they already come with preset tunes, and the tranny performance increase on noticeable. You'll see sharper shifting, it'll stay in gear longer when you do a wide open throttle take offs; if you ever do that. And if you're the type the likes to do manual shifting for gears 1 through 4, you'll be able to go up and down the gears without that " hey dickhead, that hurt's" noise from the transmission.
Don't spend too much money on one though. $400 new is a lot when they can be found on ebay for $50 to $100 unlocked. And if you do decide to change out the rear end gear, you'll need that tuner to recalibrate the car's computer for the increase in wheel revolutions.
