my suggestions for fixes that will improve the mod:
1. cleaner 3d work
fixing all the holes in the model, cleaning up the mesh especially in the interior, could also just rebuild a brand new higher quality model, iirc you stated that you have some 3d-skils so this shouldn't be a problem, also make a lower poly lod2 for better performance. the current 3d work is really messy and poor.
2. fixing the hp/tq curve
seems that you have tried to make the mod have stock specs, peak power should be at about 1000rpm higher than now, and with torque about 200rpm higher. torque isn't the big problem here but better to fix it.
3. fixing weight + weight distribution
weight of the car is off by couple hundred kg's(its supposed to be about 1450-1500kg) again i assume you are trying to make the car have more or less stock specifications, the cars weight is more on the front: 53% on front and 47% on the rear. it would make more sense for the front to be lighter than it is now so we can get the 50/50 weight distribution. i can feel it be a bit too understeery.
4. driving feel
the car on corners is very slidey and hard to control even with super tyres and lower amount of pressure for better grip. also the huge amount of downforce is just very unrealistic and makes no sense to have it that much on an m3. my suggestion is to start over in making of the physics.
