From my limited knowledge and some quick googles it appears that using non-square (but still Power of 2) textures is fine for anything other than deploying to iOS devices within Unity. This is apparently a driver issue and not hardware related... meaning iOS is more than capable of handling it, it just doesn't. Some links:…
I believe that iOS compression is different, requiring square textures. You should be fine if you are developing for PC though, which by the looks of your UVs, you are.