hmmm. i dont know, i know lot about mag. ballasts and the rules that you can bend there but with digital its a completely different ball game.
but here is what i can tell you for sure,
no the ballast does not adjust for the wattage of the bulb, they could make ballasts tha do that, but they offer full control of it to user via a switch.
if you run 100% power the bulb will receive 100% of that power so 600w on a 400w bulb, now not all bulbs are made to be able to run at higher and lower wattage's they have a tolerance limit, but note this affects several things, spectrum, bulb life, and additional heat produced, both by bulb and ballast.
so if i where you i would keep it dimmed and run the bulb as close to proper wattage as possible.