Monohull keel boats can often exceed the calculated hull speed due to several reason. First, when they are sailing down the face of a wave, or surfing, they can exceed hull speed.
Yes, that is planing... albeit in short bursts due to the hull's inefficient shape. Thats talking heavy displacement yachts, I have had many mono hulls planing for hours... the Adams 10's love it when pushed hard under kite, they just go like a great big Malibu surf board.
Second, when they are being pushed by a following current, they can exceed hull speed as their Speed Over Ground, but may not be exceeding their calculated hull speed relative to the water. Finally, some of the smaller keelboats can plane...but not too many of the larger keelboats can or will.
Agreed about the GPS thing, not a good measure of hull speed.
Most of the Sydney Hobart Fleet over 60' can plane these days, look at the Volvo 60's etc etc...