Unity 4.3 Physics 2d Change Axis

I have made a small research on Google but could not find any answer to my question since this feature is quite new.

Does anyone knows if it is possible to use new 2D Physics engine on x and z planes instead of x and y planes. I have a game uses 3D Physics currently, but we are only using 2 planes x and z.

I would like to use the new Physics 2D engine to improve performance by not calculating y axis.

This question is anwser by this other question:

Do the Unity 4.3 support top-down view?