I bought the MXO2 mini last month and the calibration is OK but it does drop the frame rate on some RAM previews.
There are many options for how the Comp is displayed. I have a 1080p monitor and when the Comp previews full screen it plays at a lower frame rate (not really dropping frames) usually around 20fps for a 720p 29.97 fps comp.
If don’t have I have After Effects display the full screen, in other words just use 720 lines of resolution instead of scaling to 1080, the frame rate is fine but the screen is both letter and pillar boxed.
This is on an 8 core Mac Pro with 16GB of RAM and the nVidia 8800 GT 512MB VRAM.
The MXO only supports 8bit display through After Effects though with Premiere and FCP it supports 10bit.