Thanks Alan.
I know my way around SketchUp, but I'm a total nitwit in how to count faces etc. I thought the file size/number of faces/navigability were all very much related to each other.
My main goal to reduce file size (or better, if I understand correctly: reduce the number of faces) is I don't want to wait 30 seconds every time I click somewhere or move the camera.
If it doesn't matter a lot for the navigability of the model, I will indeed stick to my first idea to extrude the entabulature in a separate group.
About the column: it is really not that complicated. Much less than I would want it to be anyway.
It's a reconstruction, so the idea was to make a "simple" model first and then replace these raw capitals etc later on with very detailed components, modeled from the real remains.
But thanks for the link, I guess it might be an idea to project the details as an image onto the simple capitals in stead of making an actual 3D-version. Same for the frieze.
Cheers!