I am reposting a portion of a post I wrote about the differences between the Ref. 3 and the Ref. 5 from a year or so ago.
I should also point out that the Ref. 5 does benefit, (albeit very slightly), from being placed upon a inert shelf. My friend has a Gran Prix Monaco shelving system. However, he has only one Carbon Fiber shelf, (the rest are acrylic). When the Ref. 3 was in its usual spot on this carbon fiber shelf, it sounded much closer to the Ref. 5 than when it was on the acrylic shelf and the Ref. 5 was on the carbon fiber shelf. (Yeah, go figure that it would make that much of a difference, huh?! But it did.)
I thought this might be of some help, as it is somewhat related to your post, as it goes to show that a good isolation shelf can really help, (which probably means upgraded feet can do something similar).
My two cents worth anyway.
Good Luck in your research.