I've lived with industrial style high flow showers, typical domestic showers and 3kW only. IME all are entirely adequate. I can also safely conclude the high flow ones had no real advantage.
There are many things that become fashionable and often described as 'must have.' Many of course are not, and many are later accepted by society as a whole as not. If the obsession with green continues I expect we'll all have to get used to 1kW showers.
Some boilers include a tiny hot water tank which it keeps hot. Those are the type where you can add circulation this way.
Some data I collected for comfortable temp showering: Cold winter day in November: 34C shower head, 30 on tray surface. Hot summer day: 36C shower head, 34C shower tray.
I like a perceived hotter shower > In a hotel I stayed at the other day the thermostat (detent peg) was set
It has become fashionable to mark showers in degrees C even when there is no connection between the temp markings & fact. It gives the impression of precision where no control, monitoring or precision exists. The Chinese are masters of creating false positive impressions of their products.
With a storage combi with a relatively large cylinder (40+ litres), you probably could rig a circulation loop - but might need to make mods inside the boiler if they cylinder does not already have a tapping to allow a return connection.
For normal combis, I can see two problems. Firstly the cylinder is even less accessible than that of a storage combi with less scope for adding a return connection to it, and secondly, most of the combis that feature this type of pre-heat don't do seem to do it thermostatically - they just have a timed firing every so often to keep it warm. There are only have a couple of litres of stored warm water - enough to partly offset the warm up time of the boiler when someone turns a tap on.
If you are now going to start pumping that very limited amount of warm water round the circulation loop, you are just going to dump all its heat into the pipes of the loop, leaving your store of water tepid at best. Since the boiler is not using closed loop control on the temperature of that small cylinder, it will likely stay that way until it does its next tempering cycle.
The only way I can see a circulation loop working with a normal combi would be if you return the end of the circulation loop the the mains cold inlet of the combi (after a double check valve in the cold mains feed). The circulation pump can then cause a flow of mains cold through the boiler that it can sense and fire in response to. You will be returning increasingly hot water to the mains inlet which may be a problem if the boiler has a maximum inlet water temperature specified.
You also have the issue that if the circulation pump is running on a timer, it will force the combi into DHW mode all the time the pump is running, and that will take priority over the CH - so no CH for the duration even if the DHW has reached the max set point temp causing the boiler to cycle the burner off to limit the temperature rise.
Perhaps a pump controlled from a single shot run on timer, that is triggered from a PIR in the bathroom. So when you walk in, it triggers the pump for a minute in the anticipation that hot water might be needed soon.
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