Thanks Michael,

the option didn't really do anything useful, the image I posted actually is with "Keep Corners", with the "Smooth" option it's getting weird:


The reason for SubD conversion is, that I design with SubDs for a long time - it is my way of working for about 20 years since Lightwave came up with "Hypernurbs", then I used Softimage XSI and now Blender.
In theory I could rebuild the things I do with NURBS when I'm finished, but that would make iterations much slower and tedious.

Since this is a quite common thing, I thought I ask if I'm missing something, but I guess not... ;-)

Cheers,

Tom