Unless I am misunderstanding your point, I assume you mean a combi boiler with an integrated hot water storage facility behind it all mounted on the wall?
If so, a combi like a Valliant 938 with a shift load cylinder (Valliant’s term not mine) will only deliver hot water at the incoming flow rate. The shift cylinder, if so configured, holds around 1 minutes worth of hot water at a delta T of 35 degrees. The pump in the shift cylinder is to circulate stored hot water not to deliver it.
I am not recommending the Valliant, just using it as an example ( I don’t really like large Combis’).
However, from my experience, again using the Valliant as the example, unless you have a consistent cold water flow at the boiler inlet of at least 23 lpm the boiler won’t perform to specification.
A common issue I come across large properties, where a Combi should never have been installed, is :
The toilet being flushed, hot tap at the basin turned on, then the shower - cold flow to the boiler dropping well below that required for maximum output. Unhappy customer who really wants a “hotel quality” shower.