It appears to be about as efficient as a pumped storage hydro facility (e.g. here's one in Massachusetts, built in 1970 or so - https://en.wikipedia.org/wiki/Bear_Swamp_Hydroelectric_Power...)
A gas-based design seems like it would be better at a small scale - e.g. the facility in the link has a reservoir the better part of a mile away from the turbines, and has a max output of 600 MW or so.
CO2 may actually be a good working fluid for the purpose - cheap, non-toxic except for suffocation hazard, and liquid at room temperature at semi-reasonable pressures. I'm not an expert on that sort of thing, though.
The problem with using air cooling to get it there is that the humans who run the data center have to enter and breathe the same air that's used to cool the computers, and if your working fluid gets too hot it's quite unhealthy for them. (we run our hot aisles at 100F, which is a bit toasty, and every third rack is a heat exchanger running off the chilled water lines from the outside evaporative cooler, modulo a heat exchanger to keep the bird shit out)
We're not going to be able to pump much heat into the outside world unless our working fluid is a decent amount hotter than ambient, so when it gets reasonably warm outside we need to put chillers (water-to-water AC units) in the loop, which consume energy to basically concentrate that heat into a higher-temperature exhaust. When it's really hot outside they consume quite a bit of energy.
If the entire data center was liquid cooled we could have coolant coming from the racks at a much higher temperature, and we'd be able to dump the heat outside on the hottest days without needing any chillers in the loop. As it is we have some liquid cooled racks running off the same chilled water lines as the in-aisle heat exchangers, but the coolant temp is limited by the temperature of our hot aisles, which are quite hot enough already, thank you.