I think it would take more than reliable and accurate avionics to shock me (although I'd admit to mild surprise), but however good it is, it's only as good as the basic pitot-static system. The errors caused by position / shape / (probably not) leakage / boundary layer effect on a pitot-static system are significant players in any aircraft of any size and, again in most aeroplanes, are the major factors in cockpit errors,
not the errors within the instruments themselves. So, if you don't have significant errors I'd still be very surprised - but this is no reflection on the excellence (or not) of your instruments.
If you really want a deep look at this, a good starting point would be to take the aircraft up for an hour or so when you've got very smooth conditions suitable for calibration work and to conduct an accurate calibration of the whole pitot-static system. There's a reasonable guide to how this works
here and my recommendation would be that the easiest approach is to use the racetrack method, for which you can download a data reduction form
here (at form 43).
Shout if you want a hand with any of the planning or data analysis if you want to have go at that.
Just a thought. Is there a difference in efficiency of the prop, airframe or something that changes with a different angle relative to track due to the X-W component? Does the pitot tube suffer from any effects of the same?
Theoretically since you're flying into/down wind, you're running parallel to the isobars, and wind strength may strengthen or weaken as they move closer together, or further apart. However, again you're dealing with relatively low performance aeroplanes so the odds of a significant effect within a noticeable period of time are low.
G