its not a wattage limit, its how many amps you draw to light up all the light bulbs.
Growing under cfls isnt going to be cheaper when you consider how many more you need to produce the same amount of product. 2 400 watt cabs will do what you need and not hardly put a dent in your bill.
If watts=(amps x voltage) and the CFLs and HIDs all run on 120v, how the hell is it possible for a 400w HID to use less amps then 400w of CFLs? The math doesnt work out. And we are billed (in the US anyway) based on our Kw/hour usage. So 400w/hour is 400w/hour. End of that point
No matter what his reason for going CFLs is, thats the way he went. So with that said, be careful just not to go above your cords or outlets current rating. If you use 400w of CFLs (thats the actual watts not what they are similar to in an incandescent bulb) you will draw 3.34 amps. Do not forget to add your fans into that equation. If you are in Europe then you will draw 1.9 amps (as Europe operates on a 220V system).
So unless you are using a quantity of like 43 42w bulbs, beside a mega fire hazard in that size box, you would be overpowering most house hold breakers at 15A (and most extension cords as the typical rating is 12A).
You should be ok just do the math to be certain.
Good luck and happy growing.