Assuming it's the right kind of High Pressure Sodium lamp, putting out the right PAR light spectrum for plant growth the max you should attempt in my opinion is 2.
Yield is governed by a whole load of different factors, light only being one of them and assuming they're grown in perfect conditions the most you can expect I would think is about 1/2 oz off each and then you'd be lucky.
hmm yeah half an ounce each is very little. thing is im in student accomodation so i dont think id get away with a lamp that pumps any more electricity than that. no other alternative for more lumens i take it?
but how do cfl's work? is it the same concept with power and everything just simply a stronger kind of light? if this is so howcome people dont only use these in stead of sodium?