I found out how to do a super dense mesh in ViaCAD - you can do it by right-clicking on a surface and picking "Change object type" to a mesh type, that has some controls including edge length control which can be used to make a really dense mesh object. I did that with the original blend surface and it definitely does not show the same kinds of problems as what you get from export/import.

So I'm reasonably sure that it is indeed different than the exported one and that it's probably treated as a special kind of surface in ACIS and when it gets that special treatment it's able to look nicer than it does when converted to NURBS.

Anyway, it's pretty good for me to know about this particular inter-op problem with ACIS,

- Michael