the only other reason i can think of him putting a less powerful boiler in is that if he had of put a very powerful one in then he would probably have had to run a new gas supply direct from the meter to the boiler and in 22mm or 28mm copper. this would add a lot of extra money to the price of the job so make sure he hasn't quoted you for a new gas supply and then not done it because of the less powerful boiler he's put in.
as for the mains cold losing pressure the same as the hot, i'm not really a plumber so cant say much. thinking logically though the boiler runs off the cold mains so when the boiler is being used to heat the hot water and you open a cold tap you aren't going to get as much pressure at that cold tap compared to if the boiler wasn't being used , and your also going to get less pressure at the hot tap if you've got another cold tap running dependant on which tap is being opened i.e. our old friend gravity again.
to be honest, from my experience of installers i really think you will stuggle to get any joy from yours, they usually don't want to know once they've been paid.
your only other option i know of would be to get your system converted to a mains pressure hot water system by having a hot water storeage cylinder plumbed in, water would then be heated by the combi and stored in the cylinder, if the cylinder was in the loft it would give you a brilliant flow of hot water, im no expert on these though has i haven't done any unvented hot water training yet.
i hope that makes sense, there are at least 2 other more experienced heating engineers who regulary use the site, gasman and stevegb, hopefully they will give you an answer.