Subdivide UV's? Where?!

[Pancho]

I rendered out several textures in Lightwave with subpatch UVs. If you for intance have a rectangle its corners would appear roundish on the baked uv map.

But how do I use this map in SI? There is a "smooth when subdiving" button, which unfortunately doesn't do anything. As the UVs are straight, the rounded corners on the texture are visible in the rendered SI image, as SI doesn't seem to subdivide the uv.

Where's the button or what am I doing wrong? Can't find anything except from something from 2006 which doesn't help.