yeah, the solution here requires a mix of xPercent, left, yPercent, and top
tl.to(box, 2, {xPercent:-100, left:"100%", yPercent:-100, top:"100%", force3D:true})
Here is a fork of your pen:
http://codepen.io/GreenSock/pen/axzrw
And I will be the first to agree with anyone that says CSS can drive you up a wall.
Honestly, it took quite a bit of tinkering for myself the other day while trying to figure out a very similar challenge that required this same mix of multiple properties.
Here is a top-secret Pen that we held off on releasing today as we didn't want everyone's head to explode at once:) http://codepen.io/GreenSock/pen/290c237a0f550288c84ed0312267f7ec?editors=001