Actually, if the situation were more calm, one well placed round would have stopped the car. A direct shot to the radiator wound cause the car to almost instantly overheat, causing severe engine damage, but not harming anyone in the vehicle. There is one factor, which is more important than the stability of the situation.
In order for this to be effective, one has to be precise in the aim, something this situation did not allow, however, shooting a tire would have been a viable option, but the car would careen out of control, thus causing more damage. The only remaining option is to stop the vehicle by harsher means. So, yes this situation justified the use of all of those rounds.
Shooting the radiator won't cause the car “to almost instantly overheat”. In addition the car was cold as they just got in and started it. They could have driven the car several minutes without a functioning radiator before it would have began overheating much less disabled the car.