Should be no worse than dimming any other brand of bulb but I don't know how dimming will affect the spectrum your plants receive. Could probably find a 600W ballast and bulb for cheap these days and run matching gear.
Efficiency losses, especially in a DE setup, they need the heat to run properly that's why de bulbs don't have air blown directly on them, it flows thru the fixture instead. It does work you just wont get rated efficiency