There is a research article cited in the linked article that mentions efficiency comparable to lead-acid batteries.
According to the article, the common way to do compressed air storage involves either large tanks at low pressure or small tanks at high pressure with inefficiencies in both cases. At low pressure, the tanks have to be enormous, but the electrical to electrical conversion efficiency is high. At high pressure, the tanks are small, but the heat loss during compression cause big energy loss that makes the electrical to electrical efficiency to be quite low (I think about 10-20%). The modular system has many small tanks at low pressure with some computer controlled valves. The smaller tanks help to alleviate the inefficiency due to pressure loss during emptying.
Here is a direct link to the research.