The bold portion is what I am most interested in. In GT 3 and 4, you could just by "stage 1" or "stage 2" or whatever. But each progressive stage was in all ways better than the one before it, so it was always best to simply save up and buy the biggest one. That's what ultimately bugged me about 3, 4 and 5: your particular "application" was irrelevant. Whatever car you had, that's what you had, and you couldn't do anything to customize the characteristics of the engine (you could only "add power" and the power band retained it's stock characteristics, high power bands stayed high, low ones, low, etc).
For example, (IRL), an NSX with a stroker kit will feel a lot different than an NSX with just top end work, even if they have the same max power.
I would love to see a game where the player had to make a choice between installing a stroker kit (reducing the max rpm potential) and installing a high rpm cylinder head with lighter springs and vlavetrain, etc. Finding the right balance between torque and rpm in order to maximize performance and driving style for the track in question. Then, when two NSXers meet online, their cars will almost certainly be different.