There are small on-demand electric hot water heaters that are designed to fit inside a bathroom sink cabinet, that might preheat the water, but most of them are flow-limiting - meaning they limit the flow to ensure proper heat. What you want is one that just heats to the best of its ability without impeding water flow. I suspect that the solution would involve spending more money on an on-demand tank less heater, or on a hot water tank, that would make the electric shower head obsolete.
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