Well, as I've said before, we have run regular 87 octane in our 300Es for years with no adverse effect.
Positive effect on our pocket book, though.
We've tried it both ways in both cars and can tell no difference. But, YMMV.
And maybe I'm just a cynic, but when you fill up with what you think is premium, how do you know it's really any different from what comes out of the regular 87 pump?