So you say you can't put a tube chassis under a cavalier? How about the Trans Cammer? How much mustang is still in that thing? All the touring cars are pretty much shells over tube chassis.
Sure, having a stupid-thousand horsepower car is ridiculous. But having something that was swapped and going to compete with it is fun. I built a sick challenger with race chassis and race suspension. I had to go with the stock motor to keep it at 550. It didn't win anything, but it has potential.
And you know what, it was no more of a cheat than the guys using the prototypes. If modding gets banned, then the HSC/W12 Nardo/NSXR LM Prototype, etc. better all be banned as well because they are just as much cheater car as a hybrid is. With a Prototype, you jump in, copy a tune and go win some races. Trying to find the right chassis and suspension combination to work with the challenger body and they tuning it to be competitive is more work than using a prototype.
Will it still be cheating if this becomes a feature for future iterations? I made the same assumption that modding gave an unfair advantage, but if you shoot for a target PP range, you'll find that your creation will just end up competitive rather than unbeatable.