^ That confuses me too. Many (software) upmixing profiles implement a 20ms delay and -3dB attenuation of the surrounds, which leaves me wondering how to configure hardware when playing back such an upmix and why the upmixing profile doesn’t leave it to the hardware to perform the adjustment in the first place.
If you don’t find this to be an issue, then playing the 5.1 on a 3.1 system would certainly still provide a benefit, since you’ll still get the dynamics and extra bass, just not the surroundy-ness. The stereo mix is rather less dynamic and doesn’t have much bass, so on a more powerful sound system it may seem somewhat muted in comparison.
That’s what I found when I had a 3.1 setup for a while.