It really is just a matter of cost, You can burn those 400watts if you want but you dont need to, using CFL's will suffice, then again you will also have to go buy the CFL's and lamps to do that, so it might work out about even in the short term.
What I want to know is can you give a healthy plant too much light? I dont mean too much heat I mean light. And if so what does this look like? Is there a point at which more light will not= more yeild, and if so what is this point? I can burn as much light as I want but I have only a small space to grow in, so I want to max my yeild per square foot, not worried about GPW