I followed this tutorial in Blender 2.78, but the result I got was pretty ugly, unless I subdivided my object (a rectangul…

I followed this tutorial in Blender 2.78, but the result I got was pretty ugly, unless I subdivided my object (a rectangular box) by hand, instead of with a subsurf modifier. If instead I go to Blender 2.77, the result looks fine with subsurf, however I can’t use my GPU (GTX 1070) in 2.77.

Here is how it looks in 2.78 in the 3D viewport: https://goo.gl/p1yzjO
Here is a render from 2.78: https://goo.gl/IbTRAe
Here is a render from 2.77: https://goo.gl/D2W1Ci
Here is a render from 2.78 after manually subdividing many times: https://goo.gl/PnVRgT

Any ideas what’s going on?

Thanks