I think i will try to modify the firmware to compensate the steps per mm first.

When my new bearings arrive i have to take the whole thing apart anyway. I noticed today that my housing is not exactly square, this will add up to this issue too. At the same turn i can test these motors, with the casing around it's i a pain in the butt to find good spots for my dso probes. This together with the lack of any usable reference points for measuring the travel distances or arcs between the axis, I'm close to salvaging this printer and its parts for a more engineering friendly design.

I'm still questioning myself how these slicers calculate the paths when a cricle is needed? Is there any arc compensation? When i construct a circle with 20mm outer radius and 1mm wall thickness, S3D sets two oval circles overlapping each other. The inner Circles gets pretty round with this trick but the outside... I will take a few pictures tomorrow...