As with any hack/modification, you should be very careful to be 100% sure that things are working correctly before trusting any data-transfer to it.
Drobo is great for keeping what you send it safe, but Drobo is not magic - if your system sends it corrupt data to store, it won’t un-corrupt it.
I’d do the mod, connect a standard SATA drive to it, and run extensive data-checking functions to verify that the eSATA connection is preserving data-integrity before connecting the Drobo. That way you reduce the number of variables if you run into trouble.
I have a Mac Mini too (late 2009) but I would not do it : the Drobo is intrinsically slow, and unable to saturate FW800 or even FW400. So I very much doubt you will see any performance benefit in using eSATA.