You have a heat bank with a thermostatic blending valve controlling DHW
temperature, not a combi.

Yes, but you are proposing that turning on a hot tap would produce only 6
lpm at the tap. You'd got hot water faster by warming cold water between
your buttocks.

Furthermore, the combi is likely
to be set to 60C output to allow dishwashing.
Therefore, even on quite moderate hot water
flow rates, you'll be in the temperature unstable range.

What are you on about? Oh, don't bother.

I'm saying that with the combi set for 60C, the 15 lpm spec of the boiler
(at 40C) equates to around 9.5 litres per minute. Draw any more than this
and the temperature will drop, requiring adjustment at the mixer.

The 415 only has manual flow control and auto pressure balancing. No auto
temp control to maintain a setpoint temperature, only manual by mixing hot
and cold water to user requirements.

I don't dispute that pressure balancing is an essential feature. I would not
buy a shower mixer without that feature. However, it is quite possible
(indeed, is normal) to buy a shower mixer both pressure balanced (for speed
of correction) and thermostatically controlled (for temperature
pre-selection and adjusting to varying water supply temperatures).

Christian.