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The good thing about super caps is their ability to work with any voltage, unlike a lithium battery that has a defined soc for a given capacity.
6*2.5V = 15V. Since most vehicles charge at 14.4V, I would personally find 4% headroom slightly too risky.
If instead you choose a 8 stack super-cap bank, each can be kept under 2V and a very simple, yet effective, balancing mechanism could be a standard 2.4V Zenner.
The point here is to know the knee. A 2.4V zenner starts conducting at 2.2V or below, requiring a system voltage of 13.2V or below under a 6cap arrangement (So it be on and likely burn during charging) Bump that into 8 and it will remain in the off-state well into the 17.6V. If a unbalance occurs, the cap would be kept at 2.2-2.4V - a perfectly safe voltage.
There will be a reduction in the available energy of the capacitors under the lower voltage, but reliability will be very high.
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