Your eyes perceived brightness of that 400 watt light does'nt matter..the majority of the light being produced is NOT in the PAR range..that plants actually use to power photosynthesis.

Buy a 400 watt Eye Hortilux Super HPS bulb and you will be just fine as it's got very signifigant output in the PAR range compared to many other lights, plus they are damn good bulbs and people I know swear by them for final yield. :thumbsup:

Better yet, get a 600 HPS next time...most efficient HID bulb there is lumen/par wise per watt burned and does'nt really use that much more juice.