It won't modulate the 'current draw' that's fixed by the size of the heating elements at 41 amps (9.5 kW at 230V) so once switched on, the heat output from the elements stays constant.
To control the temperature, the water flow newer showers automatically regulate the water flow and keep it constant. But the control principle is still 'less water = hotter water', and 'more water = cooler water', in the same way that you manually adjusted the flow through your older shower.
In the case of the OP, they already have a 10.5kW electric shower which they are unhappy with the performance of, so for them 9.5kW would be worse.