The cars are mostly grouped based on the real-life series they run in (pretend you don't see the Huracan ST Evo in GT3), and Turn 10 also applies their own BoP to each car the Spec series, typically in the form of Ballast, Power increase/decrease and tire width adjustments. Ideally, P.I. shouldn't mean as much, because the BoP should make it to where each cars unique characteristics, alongside the skill of the driver, are the most important factors.
That being said, what makes the M4 GT3 so strong, as far as I can tell, is that is has a lot of power and torque for its class, and delivers that power quickly and consistently and throughout most of its optimum rev range, while still having very good handling overall. I haven't compared weight between the cars, but I wouldn't be surprised if the M4 GT3 is also one of the lighter cars in Spec GT3.
Now, that does mean that the M4 is very prone to spinning its wheels when coming out of corners, but the corner-exit speed and traction it has is very good as long as you don't light up the rears. No matter what, the overall acceleration and handling garner strong results. The fastest lap times I've been seeing from seasoned players using the M4 GT3 are typically 1-3 seconds faster, sometimes more, than any other car in the category, depending on the track. Hell, I saw someone put down a low 2:19 with the M4 on Spa yesterday. I'm sure that's not the fastest-ever time on Spa in Spec GT3, but I'd say it's still a pretty insane lap time regardless.
Honestly, unless there are plans to add the Z06 GT3.R to the game, I'd love it if the C8.R was moved to the GT3 class. Hell, it spent half of its career racing GT3 cars anyways.
You should totally give the C7.R a go if you haven't. It's handling isn't as good as the C8.R, but it's still very good in its own right, while also having excellent power and top speed. It's my go-to GT2 car for Daytona, as well as tracks that are balanced between power and handling, i.e. Spa. Also the Viper GTS-R is, somewhat ironically, one of the best handling cars in Spec GT2.
Semi-related to all of that, the Forza GT Multi-Class Spec series is...something to behold. The current BoP within GT2 and GT3 creates a "fun" situation where the GT3 cars (especially the META or META-adjacent cars) are actually decently faster than the GT2 cars, but start behind them. This setup almost always guarantees a significant melee at the start of the race, especially if you're in the top-end of the GT3 grid. It's great (/heavy sarcasm).
