They will still keep developing the car like they did the 12c,one thing Frank mentioned was that he was going to see some CF wheels to see if they could be applicable to the P1,these may be added later if they can be made Mclarens standards.........The seats are standard P1 seats as far as I can see.
And yeah, especially with a car like the P1, honestly, why would you change the damn thing with "tuner"'s stuff?!?! If McLaren thought it would make sense to make the car more powerful or change something to improve aero or such, they will do so for all the owners to buy.
If you do not like the wheels, talk to McLaren and have them make custom ones, at least you know they will work fine with the car and do not cause issues anywhere.
I have yet to see a single "tuned" sports car, other than by official tuners of the company (like Brabus for Mercs) that actually does not cause an issue somewhere. Especially for cars like a McLaren, why the heck change the wheel size?! There is a REASON for why McLaren chose those wheels... and its not to be cheap or worse. Its to make the car a better ride and not cause issues.
Will never understand this... Sorry for the rant