Another way to think about it is that for the same amount of heat transferred, the flow rate of the working fluid is higher. So the boiler does not have to change its' operation. Only the circulating pump. Assuming no flow capacities are exceeded. Having a lower intrinsic heat carrying capacity of the fluid can simply be resolved by increasing flow rate. And if you design it that way, it is not a "heat transfer deficit" just design flow rates. This assumes, of course, that everything is selected for the appropriate rates and capacities.
A lower fluid heat capacity per volume/mass does not necessarily imply a change in the heat source parameters or rates of change in the heat sinks. If the flow from the boiler loop to the buffer tank is designed to match the firing rate of the boiler, all is good. And the delivery rate to loops is figured with the known thermal capacity of the working fluid. The only energy cost should be increased pumping cost. With a modulating boiler, a large thermal buffer, and the general thermal inertia of tubing embedded in concrete, and the probability of all loads demanding heat at the same time (except DHW), it is probably not a big cost impact. The controls are smart and regulate how these things work out. I would have liked to have the next generation controls, but the contractor (in a quite rational manner) purchased the equipment as soon as our contract was signed. This was to protect us from price increases. Turned out the construction took quite a bit longer than anticipated and a new generation was introduced before the actual install. I have thought about updating the control system, but it would certainly not be cost effective; cool, but not cost effective. And the vendor added a fourth wire going to the remote sensor/control units; and I have only 3. I should have asked for four just for the possibility of updating, but I just did not think of it.
I am probably (just a guess) losing more energy by having a constant flow circulator feeding the loops with a pressure bypass across source and return that if I had just used a variable speed pump controlling loop pressure as I had intended. That I will most likely correct when someone makes a variable speed pressure sensing pump that is quiet and smart. To date, all possible suitable pumps I have found are way to high capacity for this system to sit anywhere rational on their performance curve. The people making pumps do not seem to have grasped the concept that a modern well insulated structure of significant size (mine is 6K sf ft), simply does not require the flow rates of what they view as "residential" pumps. They still make pumps for 1950's houses that leak like a sieve and have 400KBTU boilers that run a lot.