I don't know too much about this, but was the car weighted correctly on the driver's side? Did he compensate for the amount of gas in the tank? Can he dial it in better to get closer? A difference between left and right will tend to pull the car towards the side of the more positive camber. But that won't happen at all times, since camber changes with driving conditions. But your mechanic is right- that is a fairly large discrepancy. What does it feel like when you drive it?