Its not the wattage, its the intensity and heat from such a bulb. 400's are like supernova in the heat dept.
I at one time was running 2-150's AND 2-250's over my 65 but I have gotten rid of almost all SPS.
I was doing it goofey, Running alternate 10k and 20k bulbs. Good color and good growth with an even looking tank.
BUT even a single 400w puts out so much heat Its hard to handle on smaller tanks. I personally wouldnt want to try and fight with the heat issues of a single monster. JMO