The theory of the point of use protector is that it shunts transients within the protector so you need a lot of joules in the rating. Basically you are just heating up an MOV or two. The ground connection is just there to provide a path for that delta. You still should be just burning off the transients that were generated within the building there though. The bulk of the incoming transients should have been stopped at the Dmark or the service disconnect panel where you have a very short grounding path. I would drive a rod there, even if I had a Ufer, just for a local ground reference (bonded to the other electrodes).
I have had 2 direct hits to the mast where my weather station is mounted and all I ever lost was the serial port in the PC that runs it. That was the first strike. After working on that line (ferrites and a "phone line" style protector in a strip), the second strike left me unscathed.
You can make some fairly good protectors yourself if you carefully match the MOV to the expected line voltage.
Hosfelt will sell you them in all sorts of different voltages and joule ratings.
