So last weekend I had a look at the situation with Ogre (related to another project and not really specifically for thrive). I found out that Ogre 2.0 isn't going to get updates and it's basically obsolete and perhaps some of the issues we had with it was caused by this. Thus if we don't switch to unreal we would have to update to Ogre 2.1 to keep up with new stuff (and that, too drops support for really old graphics cards).
Also that switch wouldn't be the simplest as Ogre is heading towards not letting users mess with rendering as much: shaders are hidden much deeper and there's a new shader block system (High Level Material System) that requires rewriting how our shaders work.
And that new version of Ogre has quite easy to use support for PBR (the same type of look that ue4 has by default) and an open source material editor: https://github.com/spookyboo/HLMSEditor. So really the biggest thing we would be missing is the full blown editor (with gui editing, asset importing etc.), which lets non-programmers add assets and other stuff to the game. So I still think this engine switch is required in order to speed up thrive development, but maybe I'm leaning a bit less towards unreal now that I've looked at the new Ogre stuff and the list of other libre game engines on github.