Yes I have thought about that, but I'm short on room and I doubt that the cap bank would make things significantly better, as the set of 650 caps I have will have a total capacitance of 108.3 farads, or a voltage drop of 5.17@ 560 amps of draw. Even if it does help with battery lifetime, I already have a spare, and I've found that I can double the life expectancy of each battery by running it through a de-sulphating process on the charger. As a side note, the 650s have a bank leakage of about 9 mA (as opposed to the 3k farad caps total leakage of 31.2 mA), so adding the cap bank would probably make my drawdown issue worse.
Instead, I'm going to use the 650 bank as a big, fast charging LED flashlight with one or more buck converters powering parallel arrays of LEDs (making near full use of the bank). Charging such a thing off the alternator shouldn't be as big of a deal, as to go from 0-~14 volts should take less than 20 seconds, and with 10x25mA LEDs, last 86 minutes. Not a bad trade, considering.
Also the spec sheet for all the variants of these caps is here:
http://www.maxwell.com/products/ultr...s_10153704.pdf