yes.

this belongs in the longboard room

Francone, I know it sounds picky, but it's just knots, not knots/hr a knot is 1 nautical mph.

Back to your question, the limitation of hull speed occurs when the hull is trying to climb over it's own bow wave. The faster it goes the higher the bow wave gets, so more power just creates more resistance. That's why the statement, it takes a lot of power to go faster than hull speed. A planning hull develops lift that enables it to climb over it's bow wave, and why you can help pumping the sail by also bouncing the board a little. and why it takes less power to stay on the plan than it does to get there.
15 to 16 knots is a lot of wind, I think he's talking about much less, maybe 5 to 8 knots, when you'll be sub planing with a 6.5 and an 8.5 will only make you go a tad faster.
Once it 15 knots you can forget hull speed you should have left that behind long ago, (as long as you have a planing hull)

Longboards DEFINITELY plane, although defining "planing" is opening a true can of worms - I've been through this with naval architects who design skiffs and yachts. The most accepted definition in the profession is something that you can't actually see in action - it can only be worked out on a computer or a test tank. But an Equipe with 8.5 should be doing what we call "planing" in a lot less than 15-16 knots - in that much wind the mast track should be all the way back and you'd be expecting to be hitting speeds in the teens, I'd think. As Plettil says, not even D2 boards are full displacement hulls; despite the rounded shape they definitely plane, as did the original Windsurfers.

The "hull speed" identified many years ago by Froude (who created that precise formula you mentioned) is not actually a hard limit - it is merely an area where the percentage of hull drag that is created by wave formation rises steeply. If I recall correctly, wave drag increases by the square of the speed. A short fat heavy hull creates so much wave drag that the steep rise in wave drag effectively forms a speed limit, as Froude's formula says.However, the effect is different with more efficient hulls that have lower wave drag. Hulls that are about 10 times as long as they are wide (like a longboard) create very small waves and therefore have very small wave drag. Therefore even if there is a steep rise in the percentage of drag that is caused by wave formation, it's no big problem - even if the drag increases by the square it's no big issue if the drag is very low to begin with.

In addition, the lift created by planing increases by the square of the speed. The skinny longboard shape isn't all that good in terms of creating planing lift, but you can still feel them start to rise on the plane pretty early; at a guess they are developing significant planing lift from about 4 knots. The planing lift is therefore reducing drag just as the "hull speed" limit comes into action, and the drop in one is cancelling out the rise in the other.

As Subsonic says, a lot of this is in HPS. Be aware that Frank didn't know much about longboards; we talked about doing some work on them together, but I never got around to it.

This is a messy answer - apologies.