7.1 vs. 5.1 - Meaningful difference?

I have a Lexicon MC-8, with a 7.1 piega speaker system. I now have cause to consider removing the back surrounds to another room and changing the system to 5.1. I toggled the MC-8 from 7 to 5 channels without noticing any difference. Do people find 7.1 a meaningful upgrade from 5.1? My rear surrounds are lesser quality than the 5.1 system, so maybe that explains the lack of impact? I'd be grateful to hear other experiences.

Thanks