Since drafting reduces the resistance, it helps more the faster you are going. On a steep downhill your speed should be determined by the amount of resistance, so even if you are free-wheeling (blue virtual speed), drafting should make you go faster (rather than allow less effort)!
However, the guy at the front is likely to be limited to a (blue) speed dependent on the steepness of the decline. So best tactic should be to draft closely to use the lowered air resistance to reach a higher speed, pass the guy at the front, who then tucks straight into the draft in turn. As soon as you hit the front your resistance will increase again and you will start to slow down, so very short turns on the front should give the best speed.
So on the steep declines I would try rolling through constantly, the lead guy peeling off and dropping back to tuck in at the back, and then move up gradually one position at a time, to take a brief turn at the front and repeat.
I've watched the pro's do this when trying to work together to maintain a break away, I wonder if it works the same in bkool land?
On a less steep decline (no blue numbers) it might make more sense to take longer turns on the front, especially for the stronger riders, but when the numbers turn blue you should tuck in at the back as soon as the other riders roll past you.
Cheers,
Geoff