Well, did my GPS checks - with the results about as expected (but good to know): -
Gauge: 100km/h (locked there via cruise control)
Menu#8 V: 96km/h
MID Speed: 96km/h
GPS #1 (Garmin 60CSx): 96km/h
GPS# 2 (Navman): 96km/h
Did the test over a 4km section of road, in both directions.
80km/h on the gauge was around 77km/h on the other devices and 50km/h sits at around 47km/h so it looks like a straight out percentage increase. Roughly... taking into account measurement errors etc.
Good to know the cars is actually very accurate, at least with the current (end-of-life) tyres.