" I wouldn’t place too much reliance on the accuracy of the mph/kph conversions displayed on the cabin IFE screens."
Any particular reason why not? I'm sure your average pocket calculator wouldn't have any problems at all with this conversion.... and the IFE map systems get their speed data directly from the INS's, IRS's or FMC's (on later versions), so the source itself is unlikely to be inaccurate.
The Airshow will hold a value for a particular (short) time, rather than constantly updating it. This would produce the greatest source of error (with the aircraft rapidly decelerating/accelerating).
Rgds.
Q.