Looks like I finally won the XT-CF20 fight with my printer :) This is a followup to this posting https://plus.google.com/u/0/+FrankHelmschrott/posts/TDWsxUn2cAp
and especially +Alex Skoruppa
may be interested in the results.
The problem seems to be a mixture of Temperature and Speed. I can't really go above 50mm/s to be reliable in priting and i have to go as high as 275°C with the E3D V6 hotend with the 0.40 first gen steel nozzle. I can go a bit higher with speed though but then the beginning of infill layers (not solid, only the intermittant infill) doesn't get printed well and the nozzle slowly collects a bit of stuff. It's not that problematic but I better avoid it. Also if i lower the temperature or slowly raise speed the infill lines start to break.
Here's some more key settings using Slic3r (didn't get any good results in S3D so far):
Filament width: 1,80 (using my caliper i get between 1,77 and 1,80 when measuring)
Extrusion width: 1
Layer height: 0,20
First layer: 0.30
Extrusion width general: 0.48
External perimeter extrusion width: 0.40
infill extrusion width: 0.48 (experimeted a lot with this in the process but found out keeping it at the "standard" 120% of the nozzle works well)
Printing speed 50mm/s for Perimeters, Infill. Solid infill, 40mm/s for external perimeters, top solid infill and support, 250mm/s travel. 30mm/s first layer.
2 Perimeters, 25% infill - both of which should vary depending on the object of course.
Hope that helps anyone.