Dinan GT...doubt that you'll get much $$ on your Dinan upgrade even from the dealer who installed them. A private sale will likely have more luck...also since you do have a GT that may also affect your re-sale/trade attempts. From what I've talked to you are amongst the minority on the 550 GT amongst 2/3 dealers I have spoken to...who are having major issues moving them. While they applaud the 4 door coupe look in other models (CLS & A7) the 5 series didn't get the others did.
If it were me? I'd not worry as much about owning the car out of warranty as long as you put away something to cover major repairs and haven't dogged the car prior and you've covered major and minor maintenance you should be okay. The $$ you'll possibly spend on maintenance is well below what you'll likely shell out on a new BMW [especially if you step up past the F10 550 or into any of the M models - BMW has lost its mind as far as pricing goes to a degree!]. And don't forget the depreciation also from reselling mods [wont get full price for those either] and then re-modding the car - must build that cost back in to for any new vehicle you buy. Unless you just need to start a new project and have tons of cash. Invest in the extended warranty, maintain the vehicle, and drive it like you stole it!