My opinion is you are worrying too much about the small things.
You are looking at figures why not speak to family and friends and see what boilers they have go and stand next to them and you will see that you will not hear them unless you are listening out for them.
On an 28kw combi there is a maximum flow rate that all manufacturers can get out using 28kw worth of gas and most manufacturers have got the most they can get on hot water rates this will still not be enough to run 2 taps at the se time without a temperature change.
You are on about the power consumption whilst idle and whilst running, instead of looking at things like that look at the cost of parts and the time it takes to change the parts, if 1 boiler saves you £100 in electric over 10 years but the parts are more expensive or harder to fit then it will lose you the money that you have saved.
To me the what anybody should be looking at is reliability, manufacturers aftercare and warranty. As you could find the perfect boiler that meets all your specifications and with a long warranty but if it goes wrong you could be without a boiler for a week instead of a day etc.
It does seem that you are being penny sensible but pound stupid.