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@BillM
I went for a nice run this morning and the area of the run had good GPS horizons and 5-6 satellites were used during the run. Subsequent viewing of the recorded track revealed position errors that were certainly greater than a few meters.
I really did not think SBAS was working and so I wanted to try using the CAS command ($PCAS15,4,FFFF*31) that I had originally used and had great results with on my 2km test area.
So, I reverted back to that command and ran the 2km test where I went up the hill on one side of the road and back down on the other side (always facing traffic).
There were 8 satellites used in this test. The distances measured were good and the plotted positions showed a significant improvement over the last run along this track (using my revised CAS command) and almost as good as the initial SBAS test. The CASIC spec uses this command ($PCAS15,4,FFFF*31) as an example, so maybe my modified command to accommodate more satellites is invalid even though there are more satellites actually in use.
There are various reasons that could explain the differences and as Gordon noted, a head-to-head test is really needed. For what it's worth, I'll continue using this iteration for the next few runs to see how things go. When plotting a run on Google maps, it is easy to see the errors.