AMuS: Ferrari FIA sensor and power advantage (in German, here a translated quote):
The competition's GPS data shows it clearly: Ferrari's advantage on the straight has disappeared since two races. Did Mercedes catch up or did Ferrari have to turn down Power? And what role does a new FIA sensor on the Ferrari play?
So maybe they did indeed lose some power to Mercedes, which would explain some of why they are struggling, relatively speaking.
Still, I also agree the car doesn't seem to have that broad setup window now (and the big rear wing might be indicative of problems getting the balance, and tyre heating, right).