Man, I begged under a 400 watt metal halide and then bought a 400-600-1000w + turbo charged (1100w) hps, and I gotta say they get a little shock from the switch, but I have definitely seen improved growth with the hps. I've only begged one under the hps, its an og kush and its still short and bushy like mine were under the metal halide, but its growing way faster than the other ones.
In all fairness to my old school magnetic core mh setup, I did have other environmental problems my first time through veg that cost me 2-3 weeks of growth, so it doesn't surprise me that the og is doing so well.
One great thing about having both tho, is that A), its more natural to the plant to have the blue spectrum initially, then the yellow spectrum for flower, and B) the switch in spectrums alone is enough to make your plants flower as long as they have begged long enough, even without flipping to 12/12. My plants begged for close to 2 months from seed and were showing pistils 3 says after I put them under the hps for 16 hours a day.