Larry,
From Wikipedia: Calibrated airspeed (CAS) is indicated airspeed, corrected for instrument error and position error.
I believe your buddie's IFR testing only checked the accuracy of the gauges. That's instrument error. We did this calibration recently in the hangar, on my new IV-P, with a water manometer, because my buddie's old P-51 tester was questionable. Distilled water and a yardstick worked pretty well. Then we had to go collect flight data, on position error, as several other responders have provided techniques.  I believe "position error" answers your original question: "What is wrong..." GPS makes this all much easier than the old tower fly-by techniques and timed courses. (Be careful because GPS may not provide a gold standard either, especially non-WAAS altitude... check SEP- spherical error probable)
  If the static (or pitot!) port lies in an area of low or high pressure, you will get an error in both the difference in pressure (airspeed) and the static pressure (altitude).  You can also get errors in the dynamic pressure from the pitot side of the system caused by the shape of the pitot tube or its location on the wing, not to mention compressibility at higher machs.. Clues may be provided by comparing the two airplanes' altitude and airspeeds. How do you know the other "welded wing" is a gold standard though? It probably needs some small correction for instrument and position error as well.
  Is there a standard among Lancair flyers for comparing pitot-static data? Where's the best placement on the fuselage? Configuration? Is it helpful to add multiple static sources? There may be a wealth of information out there to assist those of us trying to optimize or calibrate.
 In my case, I acquired N330FF in February. The builders did a beautiful job structurally and mechanically, but we are still tweaking some aerodynamic data and documentation. I have had a lot of fun with the newly installed GRT EFIS and dual ADCAHRS versatility.
Thanks
Bill Miller

I flew formation with a 320 today that had an IAS 20 kts above mine in welded wing.  The weird part was that the a/c had an IFR pitot/static transponder cert less than a month ago.  Additionally, the internal static port was found to be loose and the shop reattached it and certified the installation.  Meanwhile the airspeed indicator was sent out for check out and was yellow tagged with no problem found.