Scott,
XML files, including IFCxml are typically for exporting to analysis programs such ecotect etc. They don't include all the project geometry. Stick to the IFC.
If you read through the IFC
pdf you will see that meshes don't retain SEO relationships. They also recommend saving the mesh as a library part. You can still save it out in IFC choosing the Mesh or Site & Mesh options and then either the BREP or Geometry options. These settings affect the way other elements are translated so check the whole model.
As you noticed, the round trip comes back with the top surface material on all sides. On the plus side the mesh was converted to a library part .
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"