the only caution I would give is that the GPS-based speed-course PEC methods are usually accurate when the aircraft is flying relatively slowly and in conditions of no wind or consistent wind. However, their accuracy decreases with increased speed and height for 2 reasons: firstly, with increased speed you cover much more distance in a given time and thus variations in wind strength and direction are much more likely, thus corrupting the computed TAS and thus the computed static PEC; and secondly, at higher speeds the correlation between speed error and static PEC error gets bigger, thus any errors in the IAS data can introduce significant errors in the calculated static PEC. For example, at typical airliner cruising speeds and heights, when using the course-speed method a 1kt IAS error is equivalent to roughly a 50ft height error, which is significant when one considers that IAS readings from a mechanical ASI can easily be +/-2 kt and will have some hysteresis.
The other thing to consider carefully is use of DGPS height data. Although good DGPS height data can be accurate to only a few feet, the conversion between pressure height and DGPS height depends on meteorological data that typically has a 1-sigma rms error of 30 ft. This means that even good DGPS height data cannot really be used for PEC measurement if you are converting it to pressure height data.
Regarding your comment on accuracy of different measurement methods, by far the most accurate is the trailing static, which can be accurate to about +/- 10 ft or so. The tower fly-by method is similarly accurate, and the accuracy of both methods is determined largely by the accuracy and calibration of the pressure sensors. Next up is the pacer aircraft method, which can be accurate to about + / - 20 ft to 30 ft; the reduced accuracy is due to errors in formation keeping and the fact that you have pressure transducer errors on both the pacer and test aircraft. Any GPS or radar methods will be less accurate than the direct pressure measurement methods because you have the errors inherent in the course-speed method or the errors due to GPS height to pressure height conversion.
In summary, GPS is usually ok for GA and slow flying aircraft or helicopter PECs, but I would not recommend it for more demanding applications such as high speed or high altitude aircraft, nor for RVSM certification testing. It is no co-incidence that all recently launched new airliners (e.g. A380 and B787) have had a trailing static cone attached to the vertical stabiliser!
In answer to your question on DGPS or GPS accuracy, I would suggest you need to get the speed error to within 1 kt or less in order to calculate an accurate TAS. Also, make sure the wind is VERY light and consistent. Early morning flights in still air are definitely the way to go when doing PEC measurements.
Hope this helps.