Brian, I would generally agree but I think it is a drop ceiling. If it is a drop ceiling this might cause more problems with rattling and other noises. 
Since the upper woofer is midbass, the frequency doesn't have the energy of the larger bass waves. so it actually shouldn't cause even a drop ceiling to move or rattle. If rattling did occur, it would most probably still come from the lower woofer or PR energizing the whole room.
As Brian notes, the ceiling can create a reasonable boundary depending on how close it is to the upper woofer.
If you listen nearfield, you may also want to try the "Sonic Director" tweak I posted some time back. This will direct the midbass more cleanly and reduce any ceilng bounce or reflection problems.
If you listen farfeild, then this is a non-issue and the boundary effect will give you high impact and crisp bass. You may also have much better "standing" sonics than most.
To answer your question, I have had RM40s clients use them in 12' x 13' rooms before with claimed sonic ecstacy
I think one of the key points to room size for a speaker is reducing first reflections. By definition, the limited dispersion of the ribbons is "much" easier to fit in a smaller space before suffering from this malady.
So it stands to reason that while a huge "higher dispersion" speaker might have problems in a smaller room, the RM40 will generally have less.
One of my recent clients, (who should get his RM40s very soon) has a rather small space, and debated whether to get the RM40s for some time.
I assume he will report soon on how his system sounds in the space.
