I haven't done any tests, but based on what I'm reading, seeing, and hearing, people are going about this the wrong way.
If you turn this aid on, and drive the car the same way you normally would, then you aren't exploiting it. The reason some cars benefit and others don't in your tests is due to natural car characteristics that promote activation of the aid.
Does anyone remember Skid Force Reduction in GT5? If you turn it on and run your normal line, with your normal brake points, and normal acceleration points, you barely ever activated it or even noticed it. In fact it might even slow you down because it felt like induced understeer. But when you exploited the aid, there were massive time gains to be had, even by aliens.
How do you exploit an aid that stabilizes the car you ask? You tune it to be as loose as possible within reason. You drive deeper in the corner, crank the wheel sharper then you normally would (I mean a lot, normally I don't go past 90 degrees on my wheel, but with srf I was getting to 180 and beyond at times) and when you feel understeer, instead of letting off the throttle like normal, you stab the gas to spin the rear tires and attempt to generate some snap oversteer. But the game helps stabilize and prevent the car from spinning. You now have the car pitched into the corner, pointed at your exit, and accelerate out earlier and harder, knowing the game is going to save you.
Again, SRF was extreme, I pray Oversteer assist isn't like that, and I repeat I haven't tried it yet. My point is that, you don't tune the same way, and drive the same way, and hope to find this assist shaving seconds off your lap.
You want to purposely force the most amount of Oversteer into your car that you can barely handle, and then alter every timing mark you've learned, and override every instinct you have on the track. You have to reprogram yourself to be a completely different driver.
The best way to visualize it, is to imagine the most controlled, and smallest drift possible through an entire corner. Adding the security of being able to be heavy footed and not lose the rear end.
Think of the times you over drive into a corner, and the rear end slides out just a tiny bit and the nose stays pointed at the perfect apex, and you think, man if I just held that it coulda been a badass corner! Ever had one of those? Now imagine you could recreate that in every corner, and instead of your natural countersteer instincts, you had the confidence, to keep the car turned in, and intead accelerated through the apex and could carry your speed down the straight.
THAT is how you exploit something like this. A clean lap, that never activates the Oversteer assist, isn't how you make use of the aid. Basically you need to purposely make a mistake in every corner, so that you're using the assist at every opportunity, allowing it to control your car via a computer, with perfect inputs.
Warning: This creates terrible driving habits, has no real world application, and removes any realism the game may have had.
The 4C Gr.4 car would be my first place to start. I drove it for the first time last night in the FIA thing, and it was fast, but unstable. If I had an assist to lean on each corner entry, and to catch me on every corner exit, I assume you could save quite a bit of time.
I don't think any of these aids should be allowed in Sport Mode.