You don't know what flow rate he is getting now, so you cannot possibly
say that! I get a better flow rate from my 7500 kW shower unit

I should hope so. 7.5MW would give you 50 litres per second. Enough to fill
a bath every 2 seconds!

than several rather poor power showers (NOT gravity fed) I've used in the
past.

Well, if a power shower can't even give you 3 litres per minute, it is
malfunctioning. End of story. Normally speaking, a power shower would give
you around 3-4 times the flow rate of an electric shower. High pressure
pumps are available if that is what you prefer.

I also get exactly the temperature I want, straight away - without
endless fiddling around with the controls.

Clearly you have not used a modern mixer shower. You have two separate
controls. One sets the temperature thermostatically. The other sets the flow
rate. When you finish the shower, the temperature knob stays in the correct
place, ready for use the next time. (Excluding the Mira 415 which annoyingly
requires you to lose your setting after every shower). This is unlike 99% of
electric showers, where one control sets both flow and temperature and you
have to fiddle around endlessly trying to get the temperature right every
time you shower.

You must have unusually sensitive skin. I find such a shower an
invigorating way to start the day!

There's nothing preventing you using a multi pattern head on a power shower.
This will give you a choice everywhere from "soft" child friendly blobbles
to a punishing massage.

Christian.