First off I am not a plumber or an expert so I can only give you my view of living with a large combi in a large house.
I have a worcester bosch highflow 400 (smaller than the one you mentioned)
Mine is meant to have a max flow rate of 18l/min I measured mine and I get 12 l/min in the summer and 9 l/min in the winter (colder incoming water I assume).
This is just enough to run two taps at the same time.
For a shower I have a victorian style jobby with the shower rose above my head. This is 8 inches in diameter, so not small. No way can you describe it as a power shower experience, but it is not weak there is a lot of hot water coming out and somebody turning a tap on does not affect it.
Now running bath - obviously depends how big it is but it takes 20mins to run my bath, wich may not be acceptable to you.
Also if you do buy a highflow model they work by having a local hot water store in the combi that is kept to a high temp, usually 75C so that when demand comes they mix this water with the water that is being heated to get the temp you have set on the boiler so you get the highflow rates for a period of time. When I had my boiler installed the programmer did not control the time the hot water was on so this store was being kept at 75C 24/7. I now only have the hot water turned on at the boiler when I need hot water not 24/7 and this has reduced my gas consumption by 3.5m3 a week or about 2000Kwh per year. So make sure the boiler you buy can have the hot water timed.
Hope this helps.