Pumped storage is hard to scale down and has a fairly fixed loss. There are a bunch of other storage technologies that are at the "almost there" stage though. I personally like the Pumped Heat storage from Isentropic as a possible mainstream contender:
Piston efficiency is basically limited by leakage round the seals and friction on the bearings. Can be in the very high 90%s. The system efficiency is limited by Carnot cycle like everything else, I think; they are claiming 75% AC-AC round trip, which is impressive if true.
http://www.isentropic.co.uk/