Assuming about 300K ambient temperature (on site, above ground), 800K working temperature (store the heat at 500°C) and that the second law of thermodynamics still holds, the roundtrip efficiency can not be 70%. It must be below 40%, and not just a bit below.
I think you might be wrong but my thermal is very rusty. Where you are wrong is that you might be forgetting the over 100% efficiency in heat transfer when using a heat pump. http://energystorage.org/energy-storage/technologies/pumped-... https://www.economist.com/news/technology-quarterly/21603184... http://wordpress.ei.columbia.edu/lenfest/files/2012/11/Abstr...
You are right, I stand corrected. If instead of the ambient temperature, we have a cryogenic storage tank, getting 60% round-trip may be feasible in large scale.