I use 20/20 and Sketchup every day for just what you are trying to do. You are correct, there is a lot of excess geometry imported. The import is pretty good though. There is no other way then to export to DWG and then import into Sketchup though.
It makes for some pretty heavy models, but here at the office we keep the computers pretty high spec so we can deal with the extra geometry.
It would be a real boon if you could convert the catalogs into seperate Sketchup models automatically... Although, I think 20/20 would be rather upset about that...