I've owned a Sony Bravia 2015 set (AndroidTV) since August '15 and no, the firmware improvements have not improved anything in my opinion.
Very disappointed with this TV, so much in fact that Sony is a brand I will no longer consider purchasing with the exception of games consoles.
Everything about the software implementation is just awful. As you say, the user interface is slow and I often find the thumbnails on the home screen are pixelated for at least 10 seconds before becoming full resolution. The HDD recording is as basic as it can get and is also disabled if you choose to activate YouView and the YouView app becomes unresponsive after awhile.
You'll have a far better experience buying another TV that gives the picture quality you want and paring it up with a separate Android box or stick. Aside for the 4K, my Sony TV purchase has been a complete step backwards versus my Samsung D8000 set and Chromecast combo that I had previously.
To rub salt into the wounds, Sony just don't even seem to give a toss that they've released an unfinished product to the market. Despite the TV being released without promised and advertised features (PIP/USB HDD recording), the awful performance and poor stability, Sony have not even offered a free movie rental. PlayStation network goes down for a little while and those users received free a subscription extension.