I don’t know about the RED line becoming thicker but I get it blurried most of the time. I think is a display problem connected to the render mesh.
As for the conversion did you tried to remove the symmetry before the conversion? Does it make the geoemtry more symmetrical or it keeps the same differences? I think it’s related to the Face packing that’s not symmetrical. @pierrec any tought?
The reflection line works much better in the current Beta. You can change its size and color, and it’s a constant screen-pixel size like edges and curves.
ToNURBS SubDOptions Faces=Packed on symmetrical SubDs doesn’t currently consider the symmetry to compute the face packs, so they can show asymmetrical topologies in the output Brep. The geometry remains symmetrical in all cases. You can use SetPerFaceColorByFacePack to see what the face packing will look like in ToNURBS.
Tracked here (has to stay private, sorry): RH-64292 toNurbs output from symmetric SubD (created with reflect) is not symmetrical.