Since the introduction of 4.1/4.2 on xperia phones it seems impossible to install apps on the SD card, even with root. I mean 16GB is quite the amount but high end games these days are large (some games like beards tale are >3GB). So yes 16GB of storage is easily filled even when you can get all your movies and music/pictures on an external SD card.
I have searched and searched but it seems impossible to install apps on your SD card with Sony phones, did not manage to get it working on my Z1 and seems just as impossible on my Z1C. Any one any ideas on this? This really seems a limitation of Sony phones.
Also see, where they got a 'working' solution which did not work for me: