Yes, speedos read slightly high because it's illegal for them to read even 1% slow. Switching to a different tire size would definitely affect your readings, but not enough to be important at highway speeds. That really only comes into play if you're on the track gathering telemetry and really want to know your exact speeds, but in that case you'd be getting speeds from GPS anyway. I don't know whether BMW calibrates the speedometer differently depending on whether you ordered 19" or 20", but if not I suspect they calibrate for 20" because in that case switching to 19s means you'll be going slower than the speedo says.
'16 Cayman GT4 (delivery pics
Gone but not forgotten:
'11.75 M3 E92 Le Mans | Black Nov w/ Alum | 6MT (owned 5/2011 - 11/2015)