Hi Martin, glad to have you onboard.
USG's Fiberock Aqua-Tough Underlayment has come up a few times in the past but not much in the way of actual experience has been posted here. I personally have none. Do a search on Aqua-Tough
and read through the 6 threads mentioning this product, then decide.
In the meantime maybe someone will come by with actual experience using this gypsum based product under floor tile.
Have you considered Ditra coming in a 1/8" thick? http://www.schluter.com/english/prod...601-index.html
Ditra is what I would use and have used throughout my own house even, floors and countertops.