Talking about the GT4 version, i didn't check the Cup one, there is a issue in the HDV, probably a typo:
I think this was supposed to be -0.112.
This causes the weird physics with the aerodynamics of the car.
On top of that, destroying the front bumper is the way to go since by removing it, the car actually loses lift generation (true positive lift, not negative) and so it "gains" downforce, losing also drag and making the car faster on straights.
Basically to go fast with this car the key is to make it as more oversteery as possible (rear tires have huge grip also) and then smash the car into a wall to remove the front bumper/splitter. Are there plans to fix this, both typo and behaviour with front damages?