- Your formula is Power = 0.5 * Thrust * Exhaust velocity/efficiency.
This equation gives to the beam power for all forms of propellant based propulsion. There could be several terms for efficiency, e.g. describing losses from the engine to the beam, electrical power conversion within the engine, power conversion from an external power source (e.g. the sun) to the engine. Its similar in concept to Kinetic Energy = 0.5 mass V^2 except that power is the time derivative of energy.
- The main practical problem so far, after the mass of the power generation (solar panels), is the mass of the thruster itself. Ion thrusters have charateristically low thrust to weight ratios.
Beyond this rather hand-wavy claim the scaling-up is going to depend upon the type of ion thruster and you will need to do some digging. To some this just means the kaufman type or the RF type though some people consider these just to be gridded ion engines where the hall effect thruster is another, non-gridded, type.
Operational satellites with Electric Propulsion (from 2012)
What is the performance of ion thrusters in actual deployed spacecraft?