Interesting comments Roland...
By my understanding planing is defined by whether waterline length (wavelength of the bow wave and it's speed of travel, longer wavelength = higher speed) is determining the maximum speed. You can see when the wave shape changes and it's at relatively low speeds, particularly if you use the T foil to lift the stern. You see the bow wave rise behind the boat, rather than around the transome and to my mind this is the start of planing.
Firstly, is this analysis correct?
Uffa Fox says in his autobiography that the national 18 didn't plane but '...cut a groove in the water and ran very fast in it...', which sounds awfully like the modern Cherubs which you describe. The 18 has a relatively narrow hull for its length...beautiful boat btw. I think most modern designs are more like a N18 than a Firefly, at waterline level at least.
Destroyers/frigates plane bow down...don't they?
Born Slippy seems to go fastest downwind with the bow as close to the water as you dare...I think Loco does the same, though I'm not sure yet. Videos of many recent boats show them going very fast bow down, using T foil.
Conventional wisdom is that we get the bow out to reduce wetted area but I suspect there is also a need to get that draggy stern out...balanced by a need to stop the bow turning the boat, pitch poling.