Usually, I set the "texture" entry to "Incandescence" instead of "Diffuse" to enable the display to illuminate the scene. Then adjust the properties of the material according to my screenshot:
A big issue - especially at text rendering - can be the lack of antialiasing. Unfortunately, it's quite demanding in realtime HMD VR sessions and thus must be kept at lower settings for more complex scenes.
If only a few pixels are used to render small text and the text occupies only a small fraction of the screen, aliasing issues are even more apparent.
So I recommend to use bolder and larger text, if possible.
Keep in mind that only the high resolution area of the XR-3 is capable of displaying text in an acceptable