With the use of mains pressure from your incoming cold water main, combination boilers create instantaneous hot water. You can get a decent shower flow by fitting a shower mixer valve if the incoming cold water pressure is good. The flow rate of power showers come from a shower pump that draws cold and hot water at low pressure from a cold-water storage tank and hot water cylinder and boosts it to the shower fitting through the pump. Putting a mains pressure through a low pressure pump will damage the pump seals and lead to leaks. It is, therefore, not the case to have a combi boiler with a power shower. A power shower cannot be run off a combi boiler. You cannot have a combi if you intend on keeping your power shower since it is not possible to pump mains cold water.