You should also keep in mind that many, if not most, spikes can be generated within the facility. Motor loads, especially refrigeration/cooling seem to be big offenders. If the problem does end up being spikes, you'll likely end up putting some protection on the house panel, but I wouldn't spend a nickel until you see the results from the POCO.

Do the tenant spaces have the same type of lighting & ballasts? You mentioned the ballasts were for HID fixtures, which I will assume means something other than fluorescent. If that is the case, I don't feel that the technology for electronic HID ballasts is anywhere near being mature. Early on in the T5HO technology rollout, I ended up replacing 90% of the ballasts in one installation.

Also, if all the ballasts were from one lot, there is the possibility of a manufacturing defect that the manufacturer may not have detected as yet.

I'm definitely interested in seeing the outcome, keep us posted.

Few things are harder to put up with than the annoyance of a good example.