The point is, that a power shower is where water is supplied to the showerhead under pressure. This can be provided by a booster pump off a gravity system, or at mains pressure via a combi. So in fact, a combi shower is in effect a power shower (although probably not quite as good).
You cannot fit a pump into a high pressure system. Pumps are only designed to work with low pressure gravity systems. Also, they cannot achieve anything extra because they cannot pump more than the mains feed to the property can supply, which is available at 3 bar anyway.