We have an electric shower a Mira Advance 9kw - our water pressure is fine - but shower spray is very weak even in summer - would we benefit by upgrading to a Mira Sport Max Airboost 10.8kw.I am aware of changes between summer and winter - but I believe the Sport Max with extra wattage would help improve the strength of the spray.

The wattage is for heat, not sure it would do anything for the flow. Have you tried to clean the showerhead?

I agree. Remove the showerhead and check your flow without it. If you have a full stream, then clean the showerhead with CLR, or vinegar.

Read up on Mira Advance. It is an instant hot water heater for a shower. Specifications state that flow may be reduced in order for it to maintain the temperature
The Mira Sport Max Airboost is a 10.8kw unit and may allow a greater flow.

Note. I am not personally familiar with these products.