I'm reminded of a country show I was at some time ago...
There was a stall with well dress gent selling magnets for every occasion. Arthritis, rheumatism, migraine, barking dogs, etc, etc. And one for the fuel line on your car.
He could see my eying his product suspiciously and came over for a chat.
The claims for the device were on a list as long as your arm. Increased power, easier starting, quieter running, better fuel economy to name but a few.
He didn't like my sceptical attitude, and took umbrage to the point where he became argumentative.
He produced signed endorsements from 'delighted' customers who had experienced 'all the benefits' of the system first hand, and offered a 'water tight' money back guarantee.
I countered that many other factors could easily have caused the perceived benefits experienced by his 'independent' customers. I went to to advise him that I wouldn't be parting with thirty quid based upon his anecdotal evidence and that I'd need to see some kind of double-blind study data to be convinced. Naturally he didn't have any.
I asked what the scientific mechanism for his wonder product might be, not least as it worked equally well for petrol or diesel engines.
Further, I questioned why car manufacturers, keen to out-do the competition in these key areas, wouldn't fit such an 'inexpensive device' (his words, not mine) as standard?
He started to become a little aggressive and he told me that 'people like me made him sick'. He went on to define 'people like me' as those with 'no faith in anything and always wanted proof'. I pointed out that it is precisely that attitude that had brought us the the current point of technological advancement.
My parting comment to him was that 'he was a disgrace, separating honest people from their hard earned by peddling snake oil'.
Anyhow, enough of that.
I can see that a bank of super capacitors will lower the supply impedance to a connected device, provided (crucially) they are connected right next to the device in question. A long cable run to and from the capacitor will negate any beneficial effect.
Any well designed system will have local regulation and filtering in place, especially in the electrically hash automotive environment.
A 100+ amp pull from a starter motor will drain even the largest capacitor in an instant, so I doubt it'll help there.
A reservoir at the ignition? I could see that, especially when cranking but even ancient systems employ a ballast bypass to allow for this. Electronic ignition ought to deliver the same spark under all conditions: that's the point of it.
A (local) capacitor next to an over sized audio amp plumbed with inadequate power cabling is another possibility. It would help with current transients on heavy bass notes. But I remain sceptical.
Anyhow, if I spend any more time here I'll get the rolling pin (or the frying pan -whichever is to hand). By all means get one, and post a review of how you get on. I'm sure other members would be interested.