As of right now, there really isn't anything out there to "hop up" our cars with. Hopefully soon there will be.
I just got the standard 3 yr 36K warranty. In order to get an extended it was putting my payment a bit higher than I wanted to go. I could have lowered the payments and gotten the extended warranty by going with a 72 month loan and 2.9% APR but I decided to stick with 0%, 5 yrs and the standard warranty.
Yes, you can "sell back your waranty" as well as you gap coverage, I did this on my SRT-4 and my Tacoma, if you are still inside the 3 year 36 (for most vehicles) they will give you the full purchase price of the waranty, as for the gap it depreciates over time.
Also, to the best of my knowledge, you can make any modification after the catalytic converter (cat back exhaust), and before the throttle body (intake, filter etc.) also you can drop it up to 2 inches, pretty much anything beyond that will void a waranty, but lets say you put headers on it, and you blew up your tranny, if the aftermarket part does not have a direct relation to the part in questoin, you are pretty safe. Or you could always take the part off, like if you put a pully on it, blew your motor, and took the pully off before the dealership saw it, that usually works, unless they hook up to the pcm and start poking around there.