I also had a look at your file, sorry it's taken me a few days, not been well.
Hans,
The 10cm think floor is partly my fault, it was originally designed as a test for a first floor gallery model I was building at the time while doing the Uber and Phong shader tests. I later used the settings learned from the tests, to make a new first floor model of around 40cm. [:)]
Here is an updated image for comparison.
Are you building the model to help you learn, or is it for commercial reasons?
I ask because I didn't know if you wanted me to upload the version used for this render, or if you would prefer a step by step guide of changes made. [:)]
db4tech