Speed wise, yes. It looks like the processor in it is quite a bit more powerful then most NAS CPUs that use Atom chips. Though even an Atom doing software RAID is likely to rebuild way faster then your current Drobo.

NAS wise, the ReadyNAS Pro would be the product to compare to the MicroServer. It uses a more capable Core processor instead of the Atoms used in the ReadyNAS Ultra line.

As for expansion, can't really say myself, as I haven't kept up with the NAS distros in a while. I was under the impression FreeNAS 0.7 used ZFS as an option, but I don't have practical experience expanding ZFS.