I have done some research and quick calculations just last week. for the led's I have sitting around, I would need about 800 per square foot to reach the same lumens as a hps, that was my goal. I'm sure it could be done with 200-300 led's per plant. the light is focused pointing downward, and have specific colors that the plant uses, so it may yield ok results, but the time the project would take to put together that many led's just isn't work my time to try it. I'd rather go all out and buy a 430 watt son agro lamp!
on a side note, it is fun playing with led's. I soldered 25 blue leds in a small square. shine it in someone's eyes and they see yellow spots for the next few hours. 90% of the time, it was me looking into it by mistake. cheap amusing toys. this past weekend, I rigged a single white led in an 88 cent flash light from walmart. works very well, but I'm still gonna add 2 more to it.