The Quad 71 is really a good board at my point of view.
With 4.7 and 4.2 (it means from 20 to 30 knots), with my 67/68kg, it is perfect.
As soon as i use my 3.7 and i am overpowered, i have the feeling that i would prefer a smaller board with less volume.
Over 30 knots, it is not so easy for me to complete a bottom at full speed.
Others riders who are 70kg or a bit more do not have the same problem and control the board much more easily.
That is the reason why i am thinking to the 66l.
I remind you that my biggest sail is a 4.7.So, if the 66l supports easily a 4.7, by reading your explanations, it could be a good idea.
I remember that when i was at Moulay on last summer, when there was high wind, all the light sailors wanted to go with the 66l Evo.