Eh, there are other electric engine options that are better than ion engines, such as plasma engines or mass drivers. And if power isn't a limiting factor the ultimate in propellent efficiency is a photon rocket, aka a light bulb. No need to carry propellent, though you need 300 megawatts of photons to get a pathetic newton of thrust, so whatever reactor is powering it will need a shit ton of fuel. Though if you're using a laser array to push a mirrored sail that power plant can remain on the ground.