I don't know if I quite agree with some of this although my oceanography is very limited.
I don't think the boat is actually moving at wave speed except in certain limited conditions, because the wave motion is only passed on to the water
to a very limited extent. That is a cork moves up and down and to a limited extent back and forth.
The Jordan drogue is designed for breaking waves ie to cope with that force which is about 10 x that normally encountered.
A wave will break when its height/wavelength is > 7. Obviously when it nears shore this condition applies.
However although the Coastguard tests were done in near shore shallow conditions, one would not be using a Jordan drogue approaching a lee shore.
conditions where it would be deployed one would require around 50 knots of wind
over a long fetch to give a wave height of 10 m with a wavelength of 160 m.
In moderately deep water
this would give a wavespeed of approx 36 mph for a freak wave and half this for most.
These are not true breaking waves under the above formula although the tops may be blown off.
What I think happens is rather than reaching wave speed, when the wave is high enough the boat is lifted and then begins to accelerate under gravity and fall down the 10 m. If the boat was say 10 m long it would it would be on the wave for around 1.5 sec or rather more because it would be moving with the wave, say 2.5.
The acceleration due to gravity is 10 m/sec/sec. Being on a slope the boat in effect slides down it. My physics after all this time is a bit rough but I think the maximum speed it could reach is the same speed as if it fell the full 10 m so the slope doesnt effect the end speed rather the acceleration. So it would hit at most 20mph and probably half this because of drag say 10 plus maybe the initial speed say 7 giving 17 knots.
when it hits the trough there is some major impact of deceleration which could slew the boat if it wasnt square on which is very likely and that broach leaves it vulnerable to inevitable rolling by the next wave.
Hence I think the key is to slow the boat in ordinary circumstances ie not breaking waves as the worst possible case but in pretty extreme circumstances, and this can be done by a long drogue preferably weighted so it goes down quite deep compared with the wave height. It may be preferable to be multiple but I think lesser means than full deployment of a Jordan drogue may be generally useful.
Conversely if you actually slow the boat too much, presumably it could slide backwards down the rear of the wave and a windvane
wouldnt realise you were in reverse so turn the wrong way making you again vulnerable to a roll.