Ok, I can't really explain why the reverse is slow, because it should be half the velocity of going forward. For your second issue, I also configured it so that pressing only A and D without W turns you slowly because it resets angular velocity, while pressing A or D with W does not. This is because I noticed that turning with A or D alone will immediately flip the vehicle, so often that it becomes boring, because you are immediately applying torque to a rigidbody with no forward velocity, and based on physics, that just simply flips the rigidbody.
If you think its more fun to allow the player to retain angular velocity while just turning, I can put that back in. But for me, having my car flip by taking a small corner every time sucks.