What flow rate are you achieving now?
Increasing the hose from 11 mm (which seems a unusual size, my mira sport is 10 mm ID) to 12 mm should give 1 to 1.5 LPM increase, as long as the hose length is the same, normally 1.5 M.
Changing the shower head could give another 1 to 1.5 LPM or more, it depends on what you are happy with, I think any flow rate = or > 10 LPM is classed as a power shower.
You should be able to cut your own washer to fit the missing one or a plumbers providers should fix you up with something.
Increasing the hose from 11 mm (which seems a unusual size, my mira sport is 10 mm ID) to 12 mm should give 1 to 1.5 LPM increase, as long as the hose length is the same, normally 1.5 M.
Changing the shower head could give another 1 to 1.5 LPM or more, it depends on what you are happy with, I think any flow rate = or > 10 LPM is classed as a power shower.
You should be able to cut your own washer to fit the missing one or a plumbers providers should fix you up with something.