First there are can be vast differences between manufactures, so those numbers might be right from one manufactures, that doesn't mean they are even close for another. Second even if someone winds up getting bulbs that match those figures, that doesn't mean that a 600 watt bulb, or multiple 600 watt bulbs are the best choice for every situation. Other considerations include heat buildup, electrical costs to run the wattage, equipment costs (especial the ballast), the actual amount of space and plants being grown, the type of plants and the light requirements of the plant etc. etc. etc.. So to simply say that a 600 watt bulb is the best value, and bulb to get for every situation is a bit off. Even if every bulb manufacturer had the same specs on their bulbs as all the rest of the manufactures in the planet (witch simply isn't the case). Those extra 200 watts equal 1/3 more electrical cost as well, not to mention the added electrical cost for more ventilation because there will be 1/3 more heat from it also. If the extra wattage isn't needed, that's wasted money, as well as wasted lumens in my eyes.

No they are not as good as the specialty bulbs, but I guess the real question is what's the best value. I bought the http://www.eyehortilux.com/superhps.html Eye Hortilux super HPS. This is what they say.... " Provides 25% more energy in Violet, Blue and Green spectrum than standard High Pressure Sodium EN designation means environmentally friendly in that the lamp pass EPA, TCLP test criteria as non-hazardous waste. " I bought a 600 W because it's "supposed to be" the most energy efficient, Next I want to run as few bulbs as possible to minimize expenses short term and long term. this bulb has a 2 year life and should help with the fact that Jim not running MH. Since I'm growing peppers, MH isn't a priority for me anyway. Is the initial cost going to save me money in electricity and how often I have to replace the bulb in the long run? That's what my priorities were, Also I can toss this bulb in the garbage as it's not hazardous waste, which is also an added convenience. To be honest I don't know the right answer to your question, but I think the things I described would answer them for you. Now if I can find some research on efficiency, spectrums, length of life and factor them all together, whatever is most practical without sacrificing quality I would do.

To follow on Bus's comment, heat is proportional to wattage. I would expect that any combination of lamps summing to the maximum specified (in this case 120 watts) would be safe. An interesting side comment : a single 120 watt lamp (if such a creature existed) would produce more light than two 60 watt lamps. All other things being equal, the higher wattage lamp is more efficient at producing light. Check the lumens/Watt ratings of various wattages of 120 volt, standard service bulbs, to see what I mean. If you want to be really safe (and save some money on electricity in the bargain), fit a screw-in compact fluorescent. It will produce much less heat for a given amount of light.

Cut the power to the circuit and take off the ballast cover (the main cover). The ballast label should say what type it is. Look at the wiring diagram on the ballast and the wires themselves. Typically, with instant start wiring, the sockets have only one wire leading back to the ballast at each end of the lamp. (There may be more than one wire connected, but they go to another socket, not the ballast.) With rapid/programmed start, there are typically two wires from each socket all the way to the ballast on one end. On the other end, there are also three (typically yellow) but these are shared by all three sockets. Google will be your friend here. Look for some diagrams. If It were my garage and it is instant start, I'd change the sockets too. If the light is turned on and off, rather than turned on and left on, the lamps will last a lot longer with a program start (rapid start wiring) ballast.