The problem is that there are few 7.1 discs, most re 5.1
However, in my view of there you do play a 7.1 like War Horse, then the gains of 7.1 are substantial.
However the best improvement comes when using PLIIx music. On good recordings the sense of space and ambiance is truly astonishing. I'm never aware of the sides or rear backs, until the clapping starts or it is a program with a lot of audience participation like the Last Night of the Proms, when the audience belt out Land of Hope and Glory, Rule Britannia and Jerusalem. 10,000 people in the audience plus a large orchestra, full organ and massed choirs make an enormous sound. So all speakers are really being pushed. The one amazing thing about that such a large audience sings on time and in tune.
The other advantage is that you can play the old remastered 4.0 recordings properly. For antiphonal SACDs I use the backs not the surrounds, so it sounds correct. That alone justifies the effort.
However it does mean that for the reproduction of what I'm talking about, all speakers have to be highly capable and closely matched.