Most all boilers with indirects are wired to provide priority zone for it - this means when the tank's aquastat calls for heat on the tank (by no means every time you use some), all of its heat goes into reheating the water in the tank. Normally, this doesn't take too long, and most people never notice that the house is not getting heated for those (usually) 10-15 minutes or so. You do NOT upsize the boiler when using an indirect unless you are running something like a spa or a Laundromat where there's a call for lots of hot water constantly (and then, you'd probably supply one dedicated to that use).
That heat loss actually sounds a bit high, but might be right for your house right now if they did the calcs properly. Additional insulation might get that figure down as might new windows, so something like a 60K mod-con would still have a margin for your use. In boilers, bigger is not better.