I used to have a silver 535 and the way your car looked with the mpars was what i wanted to do with it. Other than the painted skirts and widegrill, it was there. So I definitely agree that you did it right.
But I know you've tried selling other car(s) that you put a lot of work into, so I'm sure you know that you never see more that a small fraction of that investment in return... So unless you're prepared to accept that with this car, you should keep it.