I prepared a couple of RINEX files (using programs that you can download here) generated from a GPS12 and a GPS12XL placed about 15 mts. apart and put them into a GeoGenius project along with some Broadcast Ephemeris obtained from the net.
These were the results for the baseline (DX,DY,DZ):
Then, I downloaded some EUREF data from a reference station (MAD2) about 30 Km from my position (I think it is the NASA satellite tracking facility in Robledo de Chavela, for those who know). The results, using also 10 minute data were:
The complete reports for the three processed baselines as generated directly by the postprocessing package can also be checked here (report1,report2, report3).
In conclusion, despite the low figures obtained, I wouldn't expect much more than a around-the-meter accuracy. Let's see.