well, keep in mind that you should be more worried about the bulbs output (lumens), overall, the wattage is the wrong number to stress about when it comes to growing, at least in my experience, I started out growing with 3 42w 27K bulbs from the Depot, but I was paying more attention to total watts as opposed to what is real important, lumens/ sq. ft, for example, if you have a grow room 2 x 2, 4 sq ft, you'll cover a good amount with a certain amount of lights, lumens/sq ft, but take the same lights, in a bigger area, you'll end up with overall less lumens per sq. ft. Of course I don't have any real scientific numbers to back me up, just from my own personal observations with CFLs, honestly it's worth the experiment. Try to find yourself a copy of Buds For Less, which details growing with strictly a setup of 8 High Wattage (lumens) CFL bulbs, I think he ends up with like 7 or 8 thousand lumens/sq ft, don't quote me on that, but it's worth the read to at least get an idea of the efficiency of CFLs, vs. H.I.Ds, To answer your question, I'd like to imagine that 400ws of CFLs vs. 400ws of HiDs would at least be more efficient @ a lumens per watt ratio, but never ran HiDs...