This *might* be an avenue alternative to ion engines for flights that don't stray too far from the Sun. LEO-Moon, Lagrangian Points, inner planets. And it could be combined with ion and rocket propulsion.
You can't store all the propellant at extreme pressures simply because the tank needed to contain these pressures would be extremely heavy. There's a fine balance between weight of the tank and savings on storing pressurized fuel (both energy stored as pressure and more fuel fitting in). We're at "state of art" here and can't push that much farther.
But we can afford a *tiny* extreme-pressure tank, and we have weightless unlimited solar energy at cost of fixed-size, fixed-weight solar panels.
Run the pump with solar power, gradually pressurizing the fuel to quite extreme pressures in the dedicated, tiny, very durable tank. Release it through a narrow nozzle at extreme speeds. Speed it up even more through combustion or electric field of ion propulsion. You're converting solar energy to extra delta-v with no extra fuel usage. You have just the fixed cost of the pump+buffer tank infrastructure and they can be kept really tiny, since we don't try to get a high throughput of the fuel (and have limited energy input anyway), just to increase the propellant stored energy by transforming electricity into pressure.