i find it hard to believe a 315w cmh will replace a 600hps and yield the same amount
I'm not sure that's true, either. I do believe, however, that two 315's will work roughly as well as a single 1000w lamp, perhaps not *quite* as well. It's very close from my limited experience. If the parameter is yield per kilowatt hour then CDM's beat the pants off of other HID systems entirely. They'll pay for themselves in a couple of years or so. That's most pronounced in high rate areas & for people who use A/C to shed the heat from the grow. There's 40% less total heat to shed so those growers save twice.
The trade off is that a grower needs to put more up front for CDM's even using DIY skills like I did.