Welcome Monica!
I have an Arotherm plus geat pump and am also using it for cooling, though the summers in Germany are fortunately not as hot as in Greece. I am using it with ancient (30 year old) radiators with fans installed below them. I would say I get 5°C cooling easily, I can keep my guest room at 21°C with 30°C outdoor temperatures easily, the living room gets a bit warmer but has lots of solar gain.
At least for my setup the heat pump cannot run continuously because it has too much power and transferring the heat of the room into the radiators by pushing the air through them is relatively inefficient. This means it will cycle and the flow temperature will oscillate, partially dropping below the dew point. For UFH these spikes will be less problematic because you have a lot of thermal mass to smooth them out. I think in general Vaillant does have some form of dew point control when you use their control unit but I don’t use it.
There is no real weather compensation that would increase cooling when it gets hotter, it’s more or less a fixed flow temperature setting. However this is a self limiting problem since the flow temperatures are not that low anyways and you cannot “undercool” the house (I work with a setpoint of 18°C average which drops to 12° at the start of a cycle).
I’m not really sure how well it will perform in 40°C but results at 31°C were really impressive to me. Having that extra cold air circulation due to the fans also is a nice touch in the heat.
Regarding the cost question: I am getting a COP of 5 for 15 °C flow at 30°C outside temperature. COP for coolig will always be lower but the temperature differential is much lower than in winter. At 0°C outside I need 30°C flow temperature (30 K difference), at 30°C outside and 15°C flow I only have a 15 K difference. It’s quite efficient I would say.
Here’s a bit of data on my recent experiences: