After fiddling around with the scéne I finaly got some improvement by adding a mia exposure photographic lens shader to the camera. Below you can see the result. It is far from perfect but it is getting better.
I also added a small window in de shower area to brighten up this part of the image.



Before the above render I also renderd an indirect diffuse pass. The result is an almost black image. I can only see some indirect light around the windowframe. Shouldn't there be a lot more light? Is there something I am missing?

The indirect diffuse pass: