Simon,
On the Marstrom web site they caim that downwind the M20
will lift its bow knuckle clear of the water and semi plane up to a GPS verified 30 knots.
See
http://www.sailcenter.se/administration/Boats/M20/index.aspand follow the about M20 link
Perhaps you need to contact the company about this to settle your argument.
I am sure there are many on this site that would like to see the proof of this

All the best
Gareth