Not into electric showers, though I installed one for someone using 10mm cable, a fairly long run, from the front of the house to rear upstairs bathroom, after installation I checked volt drop across the cable and it was less than 1 volt, which to me was well within spec, considering it was a 10Kw shower, however, you can never expect an electric shower to equal in flow and temperature to a gas fired shower which are typically rated at 24kw equivalent.
If you suspect your shower is performing less than before, you need to consider things like the incoming water temperature, flow, and the final temperature of the flow, and also get an electrical reading taken at the terminals at the shower in case there may be a problem with any voltage drop due to poor connections at the source and appliance, at various cutout trips, like over temperature cut out, also if it is electronically controlled via semiconductor (triacs/Thyristors/scrs) they ,may not be allowing full electric charge through, so best to check voltage at the elements, if less than the full mains apart from a few volts drop across relays, contacts, and triacs/thyristors, then you have a problem where a new shower heater could solve your problem,
again, you don't need to go into electrics as water and electrics = lethal, so unless you are competent only check it then, generally flow rate, incoming water temperature, final hot water temperature should be able to tell you if you have a problem or are these parameters within the manufacturer's specifications, google shower spec, and compare, scaling can be an issue, transfer of heat becomes inefficient, but any heat in the element must dissipate otherwise elements can become overheated and fuse, and if there are more than one element, one element may have failed.
