Joealex23 very kindly edited the McLaren P1™ GTR page and the commented that the +PP rating per part should be calculated per level and not per part.

Joe, thank you so much for your time and effort in improving the wiki!

I see where you're coming from – the total +PP per level should add up to the total +PP gained over (more on this later) – but I'm going to respectfully disagree, at least on the per part basis, and here's why: The order in which you upgrade the parts within each leve makes a difference to the +PP gained (or lost!) for that part (and the other parts).

Proof

It may not be completely clear what I'm going on about, so here are some examples and proof with the P1. Let's set everything to stock (647PP) and start upgrading each part to Stage 1, beginning with the engine:

Everything stock at 647 PP and upgrading to Stage 1 parts

Try again, starting with the Turbo

Oh dear, now what? Does upgrading the turbo to Stage 1 add 1 or 3 PP? What about the engine? Does taking that to Stage 1 add 4 or 3 PP? And the Nitrous? Should taking this to Stage 1 make the PP change by nothing or -1?

What if we begin with the Intake?

Okay, so this time let's look at the ties - does taking them to Stage 1 add 3 (as in the first two example) or only 2 PP? And now we have to decide if the turbo, bless it, is going to add 1 (example 1), 3 (example 2) or 2 (example 3) PP?

I could go on, but I got bored and I think my point is already made. So you see, it's not simple (thanks Boss Alien!).

Going through this exercise has shown one thing that your method is definitely an improvement over however, and that's the total PP for all Stage 1 parts seems to remain constant no matter which order you add the parts in. As a result of this, I have a suggestion:

Let's remove all per part PP changes from the tables!

While playing the game, I don't think I've ever needed to know how the PP rating is going to change when I add or remove a part – so why waste our time adding information on this especially when it seems to vary based on the number of other parts currently upgraded?

Since the "all-parts-upgraded-to-the-same-stage" figure seems to be constant, let's record that instead? We could add a new table to just show that, or perhaps add it into the Part Pricing Table instead?

Thanks again for your contribution, I hope this doesn't scare you off and look forward to your reply :)