Interesting topic chaps, keep it up.
I personally can see how an oversized expansion vessel could improve matters flow wise if incoming pressure is limited - but (and it is a big but) you would need to use a massive vessel at which point the costs would become prohibitive, not to mention the space required to house the vessel.
For me the maths just don't add up (and this isn't a dig at anyone, I'm just an interested bystander)
Take this example, the guy who originally posted has 1.5bar and 17litres/m
I would say he's actually looking for 30litres per minute, so the vessel must surely have to supply the additional 13litres per minute.
A nice long shower is say 15mins, so that it approx 200litres that the vessel must provide (13litres x 15), presumably then a 250 ish litre vessel would be required (the rubber bag must take up some room) and I'd have to accept that following a 15min shower the vessel would take about 10-15mins to refill itself.
Plus I'd have to find a space to fit something about as big as the megaflow it was feeding.
Roughly speaking expansion vessels seem to be about £1 per litre so it would cost £300, plus fitting and of course would probably last 5-10years before needing replacement (I assume that the rubber bag in them can't be repaired)
So as I say, I think it will work to increase flow but only for very large expansion vessels.
Oh and of course there is no way a vessel can improve his pressure, he's surely stuck with that at 1.5bar.
I'm 100% sure short of pumping there is nothing he can do about that.