Just to say, re charge speed etc, I've been running mine at 12v with a DVR2 for years and an LED from CDRC for the past nine months, and I have the lights on all the time now.
Even with an extra tooth on the g'box, the battery stays up and the ammeter shows the (low) current requirement is balanced from a shade over 30mph in top (which is an impracticable top gear speed anyway on my gearing).
With a 45W halogen (SRM belt the past 7 years, until it breaks and I get one of Mike's) it needed 45mph to to balance in top with the larger sprocket; with 35W about 38mph as per what Mike says. (That's with a mag, so no ignition demands.)
I also run Oxford 'Hot Hands' in winter, which take another 34W I think - that does require a few revs to keep things balanced; and quite a few more with a halogen bulb, natch.
Which raises the related question whether it's good to force the dynamo to run in its unregulated phase quite so much due to heavy demand - which Mike could answer better than I . . . My standard dyn's done 50K km-odd though, so can't have done much harm methinks.
This isn't the 6v:12v thread, no, but I can't think you'd have a problem with the combination Mike outlines.
(The fact that the LED units are expensive and aren't always satisfactory - I have them on three bikes and 2 aren't as good as I'd like - is another matter for yet another thread - it's here somewhere!)