The quality of aftermarket work is a function of money. You can get better-than-factory paint jobs with skilled body shops. You can get Maybach quality and better upholstery from high end custom shops. You can get nearly ANY changes you might desire with enough money. It is a bit of a minefield because you need to know and trust the quality of work the shop does. If they have built a fancier car than you want and it turned out well, that is a good sign.
Ignore F&F and Pimp My Ride, these are Hollywood jokes, as you determined.
US based customizers like Icon, Kindig Designs, or Foose can do most anything you desire. German Porsche customizers like RUF, Gemballa or Singer can make an older car nicer and faster than any modern Porsche. Mercedes has Brabus.
Remember, custom work is VERY expensive.