What Michael says is right. The internal hidden triangulation is not a concern unless it causes artefacts. Don't forget that quads are internally triangulated as well. Non-planar polys can cause an issue but only for the Phong calculator. If the vertex normals are imported, then there should be no issue.

By the way you must turn off the Smoothing Groups option in the Geometry roll out. If this is turned on, the importer will discard vertex normals and calculate smoothing groups instead.