Unity 5.2.3 is out !!

The target eye: none feature finally seems to work, sorta. If you disable mirroring in the first frame then it crashes outside of the editor. Make sure to do it in the second frame like this (confirmed it has to actually render a frame to the Rift before you can set that property, if your camera is disabled it still crashes):

 
IEnumerator Start ()
{
        yield return null;
        UnityEngine.VR.VRSettings.showDeviceView = false;
}

That code turns the start event into a coroutine automatically, and delays the execution after yield.

What is it needed for, in practice ?

Also UnityEngine.VR.VRSettings.enabled is still completely useless. Using it can cause a lot of editor crashes and setting it to false doesn't actually turn the display off or even blank it, it just shows the last rendered frame frozen in place. There doesn't appear to be any way to have an app start with the HMD turned off, short of using command line arguments which disable VR completely.

The target eye none feature is for displaying a different camera view in the main window, rather than the VR mirrored view.

I just upgraded from 5.2.0f3 -> 5.2.3f1 and performance is awful in my project. On the previous version it always ran buttery smooth but now I'm getting judder almost everywhere.

It did seem to fix shadow issues I was having though.

Also relevant:
(726878) - Android: Fixed MSAA on newer ARM Mali devices like Samsung Galaxy S6

Sad to hear about the performance issues.