Hi Brix,
Have you done any heat loss calculations, such as a U-value calculation?
As I would suggest the starting point would be to work out the heat loss for the overall property.
We had the same challenge with a wall hung pan against an internal stud walls. We have used steel (Unistrut) inside the wall to support a Geberit frame without increasing the wall thickness. Not finished yet ? I mention this in case it may be helpful.
Thanks
Craig