I started out with a 250W with an enclosed ballast, worked great. I can't say I averaged 1 OZ per plant, but I harvested about 4 OZ off my first harvest with 6 plants. It was a variety grow, pretty much whatever I could get my hands on at the time. The spindley one in the upper right corner turned out to be this crazy sativa that had 6' long branches and produced tons of little popcorn buds, I had to tie it to itself to keep it from taking over the closet! It's not about how much you get off each plant, but how you can most efficiently use the light and space you have. With a 250, your optimal space is about 4 square feet so cram whatever you can into that space. If you start with small plants, then you can use small containers and put in a bunch of them. I haven't seen it for myself, BUT I have heard that you can yeild 1/4 lb or more from a 250W light if you use the SCROG method.
Honestly, I ended up going with a 1000W HPS eventually. The buds from the 250 turned out fine, but small. I found that the light could not penetrate much more than a few inches through the tops and secondary branches so you had to keep the light real close and sacrifice growing area. It kept me happy for about 6 months tho, and I didn't have to spend a dime on any smoke!!