Is T8 Poroton only significantly better than T12 in heating costs?

The statement regarding the exterior walls does not fundamentally change. Grym even roughly calculated it on an area basis: up to a maximum of 100€ savings.

So one can say that it is not financially worthwhile to use better stones to significantly reduce heating costs.

Can one roughly say that the heating costs per year are typical for the building size? I somehow find it quite a lot now...

85 kWh/sqm are also rather high compared to new buildings - but 240 sqm of living space is also very large. We have 8300 kWh pure heating energy demand for 190 sqm. That can certainly be improved a bit more. But seriously, even €150 for heating per month doesn’t ruin you now.