The soundtracks don't necessarily have to be 7.1 for a 7.1 setup to be effective. Your receiver, using DPLIIx processing, extracts material from the side surround channels of 5.1 material(or somewhat less effectively, the front channels of 2-channel material)which would be effective when imaged in back(though maybe not as effective as actual 7.1 source material)and sends it to the back surround speakers.
The question as to whether 7.1 is a significant improvement over 5.1 depends not on room size as such, but rather if there's enough space, say at least 4', behind the listening position for a rear sound field to form. If the listening position is almost up against the wall the effect might not be especially useful.
