Oh dear .... Yes, unfortunately that is closer to a dynamic flow figure and that's just 2 outlets open, so that really would be like one decent shower running with nothing else on. The long and the short of that is when the combi's running the shower, then that cold mains dynamic figure has to supply both hot and cold - can you see where this is going?
There was a suggestion earlier about finding out what the district main can deliver, make a call to the water transporter and say you're having a problem with the cold mains and can then come out and test the mains in the street please and let you know what mains pressure and flow they find, most will do this FOC. That then lets you know what may be achievable, if a mains upgrade was considered. This may give you other options. The alternative, to give a reasonable hot and cold service, would be to stick with stored, which is no bad thing, if designed and implemented properly.
Again it really comes down to what you want or are happy to live with, what is it you are looking for from your system?