I really don't know the internal workings of the Drobo S vs. 5D. I would assume the 5D has a faster CPU and that's really what causes the bottleneck in the Drobo units.

You may be able to run a VM off a 5D without wanting to pull your hair out. I'm mostly speaking from experience of trying to do the same on a Drobo Pro with 1Gbps iSCSI connection. It was insanely slow. My Drobo S is only one generation newer than the Drobo Pro. While it's definitely a lot faster feeling, I still wouldn't want to run a VM from it unless it was something I needed to do only on rare occasions.