That's only part way up the scale towards twin 315's in terms of efficiency. It pulls at least 230w more & I seriously doubt it will grow any better.
Twin 315 systems will pay for themselves rather quickly in energy savings. 12 hrs/day x 365 days is 4380 hours/year. That translates directly to Kilowatt hours for a 1000w lamp. The average rate in the US is $0.12/Kwh, so it costs $525/ yr to run it. An 860 will cost $452, saving $73/yr. Twin 315's will cost $331/yr, saving $194/yr. Actual number are somewhat larger because of ballast inefficiency. Twin 315's win again in that regard making the price delta even greater.
If you're running A/C on the grow, % savings for that will be similar. If your electric rates are higher, the proposition is even more attractive.
If we're talking about new equipment a vert 860W system would cost ~$250. Twin 315's w/ philips lamps would cost ~$450. The latter costs ~$121 per year less to operate so you get your upfront money back in less than 2 yrs & you'll save as much every year thereafter. Reduced A/C costs, if any, just make it more attractive.
That's all based on the proposition that a 1000w HPS, an 860w CMH & twin 315 CDM's will yield very similar results, something I believe to be true.