I tried 'disabling' the rear sensor by loosely taping a a piece of paper with about the grey level of the center of the track under the rear sensor. The car stayed much better on / close to the center line in corners, and surprisingly it did stay on track at moderate speeds. At higher speeds it dove (at least in left turns) to the inside after the turn and went of often.
To me it confirms the rear sensor limits the car from cornering. I understand it's there to provide greater stability. It would be interesting to know if the level of rear sensor input can be reduced without affecting the stability too much. By the way the USB connector looks to be what's normally at the USB cable end.
My thoughts on a different system: Two front IR sensors, and a track that goes light-dark-light (or opposite..). Two sensors as the car needs to know on what side of the dark racing line it is. The track would visually be more realistic (can i mention racing line once more?), and offer up to two times the resolution. I take the car can auto steer towards the darkest section (steer towards the sensor with the lowest output). Ambient light has in principle no influence as the darkest section remains the most dark. And the car would know it's close to the desired line as the voltage difference between the two sensors starts to drops to zero as soon as one sensor crosses the darkest level.
But that would be a new car and controls, and a new track. To space the sensors out as much as possible they would probably need to be right behind each front wheel
To get around the additional stability obtained by the rear IR sensor I wonder if a gyro could not do the same and more. I'm out of my league here but believe that in RC (drift) cars the gyro takes an input from the transmitter, and translates that to a desired x degrees/second turn rate. It controls steering, and throttle, to obtain that turn rate. In Real FX the car produces it own steering input in AI mode, and could controlthe gyro