So in summer with ground water at 15 degrees, you will probably get something like 13 lpm from it of shower temperature water. During the winter that will fall to 11 ish (depending on losses in the pipe run etc). Feeding a conventional shower head that would represent a fairly decent shower.
(the 15 lpm combi I had in my last place would do a very good shower with a little spare capacity, or two "ok ish" showers at once). One shower was a Mira 88 manual valve with a slightly "water saving" head on it. It would swallow probably 8 to 12 lpm on its own. The other was a Bristain bar mixer (but on the third storey so that lost another 9' of static head (about 3.5 bar at ground level)). That would probably take up to 8 lpm at most. When both on then consumption of both would fall a bit).
It will vary with time of year.
You also need to factor in your cold main flow rate. That needs to be enough headroom over what the combi can do to ensure that cold demands from elsewhere in the house do not affect the shower too much[1]. I used to find with my setup, the cold main could only do about 18 lpm. That tended to mean that adjusting the shower temp could be tricky. Adding more cold for example, would steal a little flow from the hot, which would in turn result in its temperature rising, giving a dead spot in the controls. Sometimes you needed to overshoot the desired temperature and then edge back towards it from the other side.
[1] giving the combi first bite of the cherry from the cold main helps as well.
Erm, no I think that would be a 'kin massive soaker head ;-)
I note on some sites the phrase "Suitable for high-flow rate systems only." is included - alas no more detail. I would think an enquiry to the manufacturer would be in order.
Well, if there were expert plumbers they would probably not be working as a sales assistant! ;-)