That would give you 81.6 watts per square foot. What about 600 watt units? That is still a ton of light at 48.9 watts per square foot. 400 watts would be a bit under at 32.6 watts per square foot.
I get that but there is a point where you hit the max light a plant can utilize. I am pretty sure the 600's would be better, easier to deal with heat from and cost just over half the price to run. I don't think you would have any issue reaching max yield with 48.9 watts per square foot. Than other factors come into play like how you train the plants, the medium you grow in, your nutes, pot sizes etc..