Well, lower boil gravity does increase the amount of bittering extracted; the exact percentage I do not know, but I would imagine that 15% - 25% sounds about right.
As to the honey, I'd add it at flameout. If you add it to the boil, you'll drive off all of the delicate aromatics, you almost might as well just add plain table sugar. I'd add it at the very end of the boil, it just needs a few minutes at high temp to sanitize. Turn off the flame, add the honey, wait a minute or so and start cooling; the wort will still be at temps high enough to sanitize for the ten minutes or so that you need to kill any buggers.