I have found multiple growers getting the same results off of there 600W's as their 1K's, but with the 1K's they averaged approx a pound to 1.5 LB per 1K. I haven't found anyone pulling 2LB or more with 1 600 like I have with the 1K's. I myself have 2 1K Horti's and will probably switch out 1 if not both to 600's this summer to better control heat and reduce power consumption making for a easier to control environment IMO. I could put 3 digi 600's in there for 300000 lums and still be saving electric and equal to the 2 1K's.
Lumatek and Galaxy seem to have the best reviews from MJ growers at this point for digi's and Maryland Hydroponics carry the Lumatek for under $200 which you will find is the best price on the net.
There is also a digi 750 HPS made by Lumatek, but it is only 240V compatible and IMO still in the first stages of trial and error.
Back to the 600's though, the Lumatek Digi 600 is supposed to have with in 5% of the usable light as a traditional magnetic 1000W HPS, who knows if it's just market hype, but doesn't sound to bad to me considering that you can put them right on top of your plants without the same heat issues of the 1K's and you run only half of the electricity as a magnetic if not less and the bulbs last longer with the soft start technology as well as the higher cycling of the ballasts. One thing to remember, most digi ballasts don't run on generators because generators do not provide a constant cycle of power which the digi needs, but I came across one brand that did.
Well, hope this helps.