[brucepick]

Remember - one feature (as yet unmentioned) of having a second or much larger battery is that I should be able to have it power the car with alternator disabled when headlights aren't needed.

I estimate that a battery of about 50-66 AH capacity should be good for 2-3 hours drive time or more, which would get me through a day's commute. Would also get me through a run in the FE competitions I've been in. In those situations my lights would be off anyway, so converting them to a more efficient light source wouldn't reduce current draw.

Re. LEDs,
I did a due diligence web search for LED headlights.
I've found replacements for most smaller bulbs but not for the headlights. Nice that they can be had for the Harleys.

I'm interested in replacing smaller bulbs with LEDs but it's the 55W x 2 headlights that draw down battery voltage. Four or even six corner bulbs at 5-8W each is a relatively smaller load on the system. I believe I won't escape the need to run the 110W of headlights any time soon. Yes I could run a pair of HIDs for 70W total but it's significant $$ for a savings of 40W.

LED headlights would require a complete reflector/lens/bulb holder assembly to replace the existing one. The YouTube above shows exactly that on the Harley. This is because halogen bulbs are built with a very small filament(s) located at the focal point of the optical system. No LED of that brightness will be as small as the short filament of a halogen bulb. So the optical assembly would have to be built to deal with the particular shape of the LED sources, focusing the light appropriately. It could happen but I haven't found them available - and any set of headlight assemblies is going to cost at least $100 or more. BTDT with a regular halogen-fit assembly for this car about 2-3 years ago.