I think it really makes a difference on what the capacitor is being used for.
For example, the battery has *capacity* but is not really good receiving or discharging more than 1C.
If you drive the car and regen slowly into the battery, then there is no advantage in using capacitors.
On the other hand, if you are racing the car and need to stomp on the brakes and recover that energy, then there is no way the battery can capture that effectively, and the capacitor could help.
So the question becomes, "How do you drive the car?"