I use IE mostly and Opera Mini a little bit. What I've been wondering, do Opera Mini and UC Browser use their own rendering engine (WebKit and ???) or are they more or less skins on the Trident engine?
As this is still one of the most requested features on Windows Phone UserVoice I assume it is not yet possible or at least no one has tried to create browsers with rendering engines different from Trident. This might be due to the limited API access for native code. So Opera Mini and all the others are propably wrapping around an IE webview, yes.