The usual definition of a power shower is one that has a pump that takes both hot and cold feeds, mixes them, and produces a fast jet. You cannot generally fit these to Combi boilers because they extract so much heat out of the hot water supply system and the Combi can't keep up. However ask your plumber because your Combi may have enough ooomph to cope.
If you can't do that, you may be able to acquire more oomph from an electric shower by fitting a higher rated unit (ie, one that takes more power from the electric supply). If you have around an 8kW model, you may be able to ask a qualified electrician to fit you or 10kW model, however it may need an uprated electrical cable to supply it. This would give you a greater lashing of hot water per minute. HOWEVER you must also check that the cold water feed to it is large enough to cope.
Oh, and yes to the last question.