I can indeed confirm that using the ART runtime mode will in fact prevent you from using Screen Mirroring and Private Mode.
I switched back to Dalvik and the those features were enabled and fully functional again.I felt that using ART sped up my system performance and my phone seemed smoother, but it could just be a placebo effect.
Is ART really THAT much better than Dalvik? Or is it marginally better? Perhaps, better on some phone than others?