Thanks for the update zefir.
I have both a level shifter and the option to use an inline resistor to drop the 3.3V signal down to 1.8V on my test board similarly to how some Bitfury designs have implemented it. Have you investigated doing that, or just feeding 3.3V straight in?
I have all these three options on the prototype board:)
The datasheet is less then conclusive I'm afraid.