Ejection speed is already high with ion engines. If desired, it could be increased further just with a higher acceleration voltage. This is not generally sought, because electric power is the limit of these thrusters, more than the consumption of propellant.More: the energy wasted in ionizing the propellant is a very serious limit, which explains why propellants are chosen heavy and easy to ionize. Removing more electrons would go in the wrong direction.
...on the other hand, if you could store highly-ionized fuel, the recombination with electrons... potential thrust mechanism.