I agree that a car setup/vehicle dynamics tutorial - if implemented properly - could be very helpful. I'm not sure that laptime would necessarily provide the best measure of setup success, however, as adjustments are often made for drivability/driving style reasons. More importantly, although the sum of numerous small adjustments may provide a consistent driver with as much as 1s, recognizing - and measuring - the contributions of each individual change through laptime deltas would be next to impossible with all but the most consistent drivers.
That said, a tutorial mode able to help isolate - and recognize - specific car characteristics would be uniquely beneficial. I've understood vehicle dynamics for some time, but have had great difficulty applying that knowledge effectively, simply because I have trouble discerning subtle car characteristics.
If the rear is sliding under braking, I'll move the brake bias forward, or increase the differential lock on coast, and if I'm spinning my rear wheels, or losing the rear, on exit, I'll lower the lock on power, soften the rear springs, or increase negative toe-in at the rear, but I have great difficulty recognizing subtler cornering characteristics (e.g. mid-corner behavior. And if the general car behavior isn't ridiculously obvious, I'm unable to make effective adjustments at all. Therefore, anything that would help me better understand car behavior is welcomed.
Of course, consistency is both the fundamental prerequisite for effective car setup, and perhaps the most difficult. Obviously, without consistent lapping, judging an adjustment's effectiveness is impossible, but consistency at every corner, lap after lap, is also necessary to understand a given car's handling characteristics, especially at a granular level. I think this is responsible for much of the black art aspect of car setup.
Unfortunately, as I've all too frequently seen first hand, the consistency required for effective car setup is - so far - beyond my ability.
Henk