First, you have a way to store an absurd quantity of positrons. For a sense of scale, without shielding the electric fields, 1 picogram of positrons (or electrons) confined within a 10 cm radius is going to trigger positron-electron pairs formation, thanks to free electrons in the area responding to the surface potential.
Second, AFAICT if you can do that then you're either going to want to use them as an energy source to propel your cheap reaction mass even harder than a fission rocket would, or you're going to want to react them with electrons to make a photon rocket.
If you're getting them from radioactive decay, it works, but those examples are 63 and 26 times heavier than just using hydrogen as your reaction mass in the first place :)
Second, AFAICT if you can do that then you're either going to want to use them as an energy source to propel your cheap reaction mass even harder than a fission rocket would, or you're going to want to react them with electrons to make a photon rocket.