Hard to say. I do know a guy that uses a 4 bulb 40 watt 4 foot florescent light fixture and he can get 2 to 3 ounces per plant on a scrog. I can usually get around 5 to 6 per plant if I scrog it with relative ease, so in my limited experience in comparison of the two, it's about double.
Then again so is the lighting. The MH bulb on the 600 pumps out 52,000 lumens and the HPS runs 91,000, so there you go.