Just using PV = nRT, if temperature stays the same, 90 cu.m at 1 Bar is about .3 cu/m. at 300 Bars (90/300), or about 2 ft x 2 ft. x 2 ft. Reasonable enough.
Actually compressed air is an excellent way to store and tap energy, similar to, and almost exactly the same as, heat, which is why a lurker like me speaks up, and why the Indians consider it. Pressure (N/sq.m) is also energy per cubic meter (N-m/cu.m.). Altho like heat and the Second Law, there has to be a sink, (lower pressure) for the energy to flow. In this case 300 Bars and decreasing, to one Bar atmosphere, a calculus problem in regulating the volume flow for a constant energy flow (power).
Equating this pressure (300 Bars) and volume (.3 cu.m.) results in a stored energy of about 2.5 kwh. That's about what? 3 to 4 12 v. batteries. Not a lot but more than enough to get around the golf course a couple times.
None of this of course has to do with acquiring the pressure in the first place, about which I can offer no numbers.
No it is not. When compressing air, a lot of energy is transformed into heat, which is dissipated without recovery making the efficiency very poor.