So the whole performance parts thing is still just a little bit vague to me - here's what I'm thinking:
If you complete a Mission, Skill , or otherwise Level Up, you get rewarded with a performance part. I assume that this process:
1) Can be equipped to your current car immediately for free, or sent to HQ to be applied to one other car for free later, and
2) Unlocks that part for other cars, but requires in-game purchase (Bucks).
So if you have to wait to unlock a performance part when buying it as a micro-transaction (real life money), I personally don't really see the point.
Unless say you spend all your in-game money on a super-expensive Lamborghini when you unlock it, and have no money left to tune it to beat others online (assuming the relevant performance parts are unlocked but too expensive). Which I fear is a possibility. It just better not be priced so high to force grinding for months or micro-transaction to the same level as 'free to play' games like Race Room Experience or Auto Club Revolution.
That said, by the time I reach supercar levels I'll probably be happy to just get the missions out of the way then just use them for cruising, assuming they have enjoyable handling.