well 650S is a 12C with a barely mild update. Basically McLaren took the 12C and gave it some P1 touches. Notice how the front end it basically a P1's grafted on the front of a 12C. The hood remains identical to the 12C.
Profile really shows the similarities.
but the rear is where the two become indistinguishable..
As for trickle down, I know the 650S makes the P1's CF seats available...
I just wish more of the P1's design language was mixed into the 650S, but they do have to keep the P1 design a bit exclusive to it. If there's one thing they can do to the lower end models is add more curves.