The problem with high wattage single-lamp LEDs is that LEDs cast a very focused footprint compared to HID lights, casting light down into a circular area rather than casting light outward evenly. You don't have the same penetration when using 300 different 3w LEDs that you get from 1 single 600w light-source, either. So if you're growing in a 4'x4' with LEDs you really want to use multiple lamps so that the spread of the light footprints cover the space more evenly.
My plants grow in 5 gallon drums so trying to use a single light-source LED is out of the question.