Have you ever seen those funny fuzzy dark lines in some pictures week HID lights? That's because those lamps are driven by magnetic ballasts, running on 60Hz AC. The lamps are actually flickering, and those dark lines represent the time it spends NOT making light!
The 315W CMH lights are driven at a similar low frequency, but in this case the ballast generates a square wave. Long story short, this one (relatively expensive) change in ballast design is responsible for all of the increased performance of the technology, by eliminating all that time the lamp spends not running at peak output. No more dark lines means much more light for the very same watts and thus higher efficiency.
I'm hearing of several manufacturers coming out with low frequency square wave or LFSW ballasts that run at 1000W. I'm about to hook an 860W CDM Allstart lamp up to one in the next few days...
Anyone interested in seeing what happens next?