I will join the masses and say wipe your arse with the warranty, I did 2 years ago and my car is still running great at 101,000kms and flew through it's 100k service with no extra work being required. The biggest thing is to enjoy your car and modify it how you want to. Also 6 monthly servicing...fuck that!
WRX's were made to be modified, they are barges (relatively) without decent sways & coilovers and they sound distinctly average without a
TBE. I do however agree that you should always keep the original parts except possibly the exhaust.