The output from a pump is always a compromise and a balance between pressure and flow provided by the electrical energy you put into the pump.
If a fixed amount of energy is imparted into the water it can increase pressure and it can increase flow and for every application there is an optimum pressure/flow ratio.
In order to give a strong spray pattern a WRightChoice Shower head combined with a single SPB will lose 2.3 metres head in the shower head to create a fast/strong spay pattern and deliver 5.9 litres of water.
Remember that showers mix hot and cold so you will exceed the output of the hot alone in a normal home, but your shower head may not be as good as ours, you may a mixer that is designed for Central European high pressure systems. If you do have problems you can double boost the hot with a SP21S or balance flow with a SP22S.
For those who understand pump curves the flow to head results achieved with a single pump is published below, just click on the link