It looks like the 2014 version of the Y6 only list one prop size. Are they not running the 2 different sizes top and bottom like the A model? Why?
We found improvements in the yaw stability, note that the motor order also changed.
Here you can find some post about this subject:
Running Different Props top and bottom was not efficient and the different sizes caused yaw and pitch / roll coupling. There is a pretty good discussion about it on the DIY forum