Sideloading apps on the Fire TV stick is hit or miss, but mostly miss. I sideloaded the TWC TV app on it, but it would fail when loading. As far as the mirroring goes, it always out of ratio like it's zoomed in a little, which cuts off a small portion of the display edges. And it also lags. Not unbearable lag, but very noticable. Other than that, the Fire TV stick is very impressive.
The lag was why I stopped using the Fire stick and now use the Fire TV box. So much faster, smoother, better wifi connection, voice search is a nice touch too.
Plex works perfectly with the Fire TV box, although it has to do a refresh to my NAS server about once a day, which means waiting a few extra seconds to load my entire libraries metadata (at least that is what appears to be happening. Not sure if it is the Plex app or my connection, but the impact is minimal, so don't really care.)
Still have to use Roku 3 or the Chromecast though for Google Play movies and phone mirroring though, which is the only flaw with the Fire TV.