Whether its a waste of power would depend on what the mag9.5 pulls when it is throttled down to match the flow rate of a Mag5. As you throttle them down with a valve, the energy usage will drop as well. Not sure a Mag3 or a Mag5 can handle the head pressure he has without choking the pump out to the point of very little flow through the sump.
If the mag9.5 needed to be throttled down, it could be used in the future to run reactors with the extra flow capacity of the pump. Its always better to have more pump than you need than not enough pump.