Assuming a horizontal accuracy of 75', this leads to an altitude error of 112'. So in theory you GPS altitude could be in error by 685'.
Just to clarify myself
Should be noted that post selective availability most of the time GPS is highly accurate, and that:-
For horizontal accuracy you can expect:
within 2.5m 50% of the time
within 3.8m 68.27% of the time
within within 7.0m 95.45% of the time
within 9.8m 99.73% of the time
For vertical nav:
within 5.6m 50% of the time
within 7.4m 68.27% of the time
within 14.4m 95.45% of the time
within 21.3m 99.73% of the time
Around the UK the difference between the geodetic altitude and MSL is not very great, though some places like the Indian Ocean show much greater deviation (-105m). So most of the time, GPS altitude will closely resemble pressure altitude (+/- 50 feet or so). Add the max error of 21.3m into it and you could possibly show a deviation of 100' or so.
Adding in ground based reference stations, and altitude accuracy increases to:
68.27% confidence: 2.7 meters
95.45% confidence: 6.4 meters
99.73% confidence: 12.0 meters
which is probably better than a pressure altimeter.
Cheers
EA