IMHO, you really don't need to do anything more, since your installation would be NEC compliant as is. Just be sure to bond the buildings interior cold water piping with a bonding jumper sized according to Table 250.66, which I'm sure you have probably already done anyway.
If for some reason you're uncomfortable with the current setup, you could always install ground plates or even additional ground rods until the cows come home, but I don't know of any NEC requirement for additional supplemental grounding beyond what you already have for this 208V service.
