Advise on combi boiler quotes

leo25

Have recently had some quotes for various combi boilers to replace my existing open vented boiler with tanks. I was looking to maximise water flow rates to power shower levels and after looking around decided to go for either the valliant 938 storage combi plus the installation of two mixer showers for £3300 or
a Worcester Highflow floorstanding 440 for £4600 both installs include system clean filter and wireless control panel.Are these quotes excessive?Has anyone
got/installed these boilers and what is the shower performance/duration like. One of the plumbers pointed out the higher risk of water leaks when doing this type of conversion should I be worried? Also Had a quote for an ideal 40kw boiler for £2800 complete with three new radiators could this be a better option? All 3 quotes also included fitting thermostatic radiator valves to 11 radiators and incoming flow rate at kitchen cold tap is 18-19 litres per minute.

leo25

Change of plan now going for 300 litre unvented cylinder in garage combined with new system boiler 24/28kw any comments appreciated.
2.25 bars of pressure recorded on Friday via outside tap combined
with 20 litre plus per min cold flow rate