After racing with the three cars, I can confirm each of them has its own distinctive temperament. The Audi is smooth as butter, IMSA like as you say, and definitely the easiest of the three. The BMW is very nimble, and I'm fairly sure it beats the other two on a twisty track. All the responsiveness, however, means you end up being very close to the limit at all times. Another complication is the occasional curveball thrown by the bug-susceptible nature of the car. To me, the BMW is the most challenging of the set. Lastly, the Mercedes feels more like a typical "slow car", and is probably the one for which the conventional driving techniques translate more easily.
Thanks, good to hear feedback like that.
I think there's a few small holes in the wiki, but I couldn't really point them out. It's mostly in the 3D model stuff I think, the description of how the coordinates of a wheel work is for instance wrong, as far as I can tell. RES-file wise I've also got some maybe new insights in length and handling I've yet to post. I'll get around to that one of these days.
Looking forward to it 
I need to write a proper post with exact numbers and add the example cars so people can try it for themselves. The gist of it was this: I got two cars with the same handling/length, but one twice as long as the other. The short one steers a lot faster, more responsive. I figure it's related to the reason longer cars jump farther: When a Stunts car turns it doesn't turn by a certain number of degrees per 0.05 seconds, it turns by a certain distance per 0.05 seconds. For a shorter car the front end moving the same distance means it turning a larger angle, so it steers more quickly. The handling parameter seems to be in control more of how long you can keep cornering before the car starts slipping. So they still counteract each other, if you corner slower but can keep it up longer you can corner sort of around the same distance at the same speed.
I think...
Because this is where it gets a little weirder. All of the faster cars can do a single wide corner at around 150 miles an hour, despite their length or exact handling parameter. The superkart does it, the IMSA cars do it, the Indy does it. It's some sort of ceiling, a limit to cornering ability similar to the limit there is to top speed. I found this limit, or the general area of it, for a test version of the Mercedes. And that's where something weird happened. I had four versions of the car. The one with the lowest handling parameter was the slowest around a lap, by a small margin, but it felt quite good to drive. You had some room for error. The three versions above it had roughly the same lap times, but I think it was the second slowest one that usually took more tries than the others not to spin out. When you spun, you spun, no coming back from it. The car was more on edge. When the handling parameter rose further this behavior went away again. I dubbed this place "the sour spot", and I tried to use it on the DTM cars by putting all of them just below that spot. The idea is that the car with the least amount of handling gives the most warning when something is about to go wrong, creating a balance between where the edge of the car's ability is and how well a human driver can stick to that edge. The concept does not completely match your description above though. The BMW has the lowest handling and even the lowest handling/length while being the shortest. To me it feels short and responsive, maybe a little too much so, but I also get pretty good feedback, which is probably part of the reason I did my fastest NoRH lap with it. It sits the furthest from most other cars in feeling due to being so short, but that was less of an obstacle for me after all the testing I did with these. The Mercedes and the Audi have the same handling/length, so the longer Audi has the best handling stat, but it feels a little slower and smoother steering-wise than the others. The Audi to me also feels a little closer to the sour spot, it handles quite well, but I have trouble finding the edge before crossing it. I made quite a few NoRH attempts with that one.
My experience was probably tainted by the design ideas I tried to implement though. There's probably some wishful thinking involved.
That's basically it. I have no idea if the sour spot is even available for all lengths of car, or that it's a artifact of a specific unlucky combination, but I definitely feel like there's a limit to how much handling a car can handle before more handling stops doing it any good. The superkart for instance is massively over-handlinged. It would drive just as well with maybe as little as half its number or more. Have to look the proper number up to make a better estimate.
The torque curve itself has some weird features, it's hard to design to an exact top speed [...]
Do you mean flat track top speed, or real top speed, or both?
The combination, kind of. The BMW has a slightly lower max rpm than the other two with the same top gear ratio, yet I could not for the life of me get its real top speed (the highest of the tree) down just one or two mph, at the very least not without altering the air drag. I could give it slowdowns or lower its flat track top speed, but I think I could only lower that by 2 mph, not one. I think one of the things that's happening is that because the game works in discrete intervals of 0.05 seconds you don't actually touch every bar in the torque curve. You often end up skipping a bunch. That's also what causes the slowdowns I figure. You skip ahead to an rpm that has a really low power (just 2 ticks or so) the game makes its new calculations with that very low torque and drops you down quite a few rpm's on the next time step. But the problem with the top speeds is that it's not one or two bars that decide a particular top speed, just the ones that don't have enough power to speed up any further. If the bars lower on the curve are too high you can skip right over those two. High air drag helps, it makes the high speed behavior of a car more predictable, and a power curve that slopes down over a longer way, giving it a big gap between flat track and real top speed, probably helps as well in that regard. Or you should just not care too much about specific numbers and exact balance.
One tiny issue I noticed is that the stripes over the hood from the BMW's paint job #9 flicker a lot. I'm not sure about what, if anything, can be done about it (I don't see anything obviously problematic when looking at the model on Stressed, though I didn't actually attempt tweaking it yet).
I had some problems with that earlier. I didn't pay a lot of attention to it post release, but I'll happily believe it. I'm not sure what can be done either. I think part of the story was interference from the wheels and the sort of wheel houses. Since the BMW has the lowest hood it's the easiest to interfere with. The final form those wheelhouses take is pretty trimmed down but I also didn't want to leave them out completely.
