So it wouldn't be limited in speed by the fuel it carried on-board. Assuming Alpha Centauri, in the later case it would coast for about eight years. At low speed interplanetary trips, the drive would work like a conventional ion rocket or mass driver. Stored reaction mass would be feed into the electromagnetic or electrostatic accelerator core. Power would come from fusion reactors running off stored fuel.
Specific impulse varies depending on the exhaust velocity of the expended mass.
Now for the bad news - slowing down. We can't pre-load the deceleration course track with fuel at the target star because it would be virtually impossible across interstellar distances. Getting out of the starsystem and back up to speed would also be a problem. If we decelerated using stored fuel. We know we can get back up to speed by refueling in the target star system.