CAES has lower efficiency, probably closer to 70% and only if you use something to reheat the air. (Compressed air loses temperature and needs an external source of heat to restore it to full volume)
Even if you use natural gas as a reheating element, the compressed air stores a significant amount of energy and is doing the majority of the work.
I too was wondering that as the ideal gas law states that PV = nRT. When you compress a gas it gets hotter, conversely, allowing it to decompress cools it. I assume they store the heat energy...
If you ignore efficiency then you stop making a profit pretty quickly. You can solely focus all you want on cost; sooner or later efficiency will make you pay.
Goldman wouldn't be making that kind of an investment if they didn't have a handle on both parts of the equation. Indeed efficiency (dealing with losses from seepage) have been the nut that hasn't been cracked with compressed air storage; these guys seem to have an economical (efficient) way to deal with that problem. If true, then they really are worthy of Goldman's investment.
Even if you use natural gas as a reheating element, the compressed air stores a significant amount of energy and is doing the majority of the work.