EDIT: Just tested with xfwm's compositing totally disabled. STILL no tearing on xmir! For this reason I think xmir could actually be a big advantage to xubuntu and lubuntu, because right now they suffer from tearing issues unless you use a 3rd party opengl compositor. And just to make sure I looked at /var/log/Xorg.0.log to ensure that intel's "TearFree" was disabled, and it was, so it was unity-system-compositor/xmir providing the tear-free. I'm glad xubuntu is at least considering xmir, unlike lubuntu.