The light saturation point is dependent on other limiting factors: Nutrient availability, environment, and CO2 levels. Providing you have a good environment (correct temp and humidty) and the proper levels of nutrients, available CO2 is usually the limiting factor. I don't use CO2, but I have good air circulation and a good extraction fan, so I'm pretty certain my CO2 levels are probably around 350ppm (which is approx. the concentration of CO2 in the atmosphere). I hit my light saturation point when my two air cooled 600w HPS's are around 14" from the tops of the plants. How do I know this? The top leaves of the middle plants (plants directly under the light) start to bleach out when the lights get much closer than this. This is fine anyway, as 14" is where the best combination of intensity and coverage is achieved for me.
So long story short, all else being equal, photosynthesis is limited to how much CO2 is present in the atmosphere. With the addition of 1500ppm of CO2 in the atmosphere, the plants can use a lot more light than at 350ppm. So I'd say ballpark figure, go with 50w/sq.ft if you rely on atmospheric CO2, and up to 100w/sq.ft. when additional CO2 is supplied (ie. 1500ppm)