It'll crash for the vanilla clients if car.ini header is using extended-2. In a way, they will be required to have it.
IMO it's a complete deadend to try to dev stuff for vanilla anymore. Especially racecars. You can get a reasonable result in vanilla but it is much more enjoyable, faster and the end result is more accurate if you dev for extended physics. Although being an axle rear car, this one won't be able to benefit from the entirety of extended features, while also not suffering some issues that vanilla DWB has, so it balances out somewhat.
Although I did look at the car and there are appear to be more physics issues than just inaccurate heat which will make telemetry/butt-dyno correlation difficult.
Kind of a good example why "driver feedback" means almost nothing most of the time, and how people can easily trick themselves. So I don't even even advertise it anymore as a mark of quality.
Getting people to measure things for you is more valuable than getting them to drive your car, because everyone will say different things about the same car, some more on point than others. Rarely do people completely muck up a measurement.
I know everybody is gonna get annoyed at me saying that, but it's the truth.