For a buffer to work, it necessarily needs to be hotter than supply temperature, which means more losses and more work for the heat pump.
Since heat pump can easily adjust the supply temperature, the most efficient system will be the one where the energy delivered to underfloor heating or radiators is just balanced out by heat loss of the building.
Where buffers are useful is heat sources whose power cannot be modulated (my old diesel furnace for example- when it fired, it produced roughly 20kW, with the only adjustment being supply temperature at which it turns off), or where there is a cost to frequent starting/stopping the heat source (I.e. you probably don't want to get up in the middle of night to add wood to a wood-fired furnace, and pellet-fired furnace needs to be maintained more often in such operation mode).
If one really needs 25kW heat source, one really needs a 25kW heat source. There is no clever way around it. The real question is whether one really needs 25kW heat source (which can be answered definitively by either measuring the delivered==required power, looking at the peak continuous fuel consumption (let's say the coldest day or week), or by having accurate heat loss model of the building. In a heating season, the buffer cannot realistically be large enough to significantly reduce the power requirement. For example, 1m3 of hot water when used as a buffer can store roughly 30kWh (when heated 25 degC above supply temperature). My house needed 240kWh of energy in the coldest day, so the buffer would only cover 10% of the needed heat.
I had set up monitoring for my diesel furnace, so I could see for myself what was the peak heat requirement over two heating seasons (10kW continuous in the coldest winter day). So, I selected an 11kW heat pump, which, if needed also has 9kW of resistive backup heaters. It also matched the numbers in energy-efficiency certificate of the house (theoretical calculation based on materials/thickness/local outside temperatures) within 10-15% or so.