I don't know how you say this. For skiffs and many dinghies, of course what you say is true, but I wasn't referring to them.
I am positive that you of all people know that a keelboat (unless crew weight is a large percentage of total weight) will in fact heel when there's much breeze, that for best speed one doesn't trim and adjust amount of sail carried to maintain overly-low let alone flat heel but rather best speed will be with some significant heel, that the boat can be designed to offer reduced wetted area when heeled and usually is designed to do this, that when shaping the hull for best speed while actually sailing this surely would have to be with heel taken into account, etc.
If you want I could find a specific boat for which a specific value has been given. But I am guessing that most likely it is now clear what I was meaning so there may be no need for a particular example (I'd really guess not as you have vast experience of course.)
Estaban, What Bob said is right (of course)
. What you are talking about is another thing: When the boat heels generates a righting moment, initially ballast counts very little for it and RM comes almost all from form stability but when the boat heels more and more, ballast starts to count on the RM.
Max righting moment normally happens around 60º and that means that it is at that point that the boat has more power to carry sails. We know that no boat sails well at 60º of heel and that it will sail faster at lower angles of heel.
Why? Because the drag is so much that will not compensate the more power (righting moment) that the boat can generate with all that heel.
What you are talking about is about the heel angle that represents the better compromise between power (RM) and drag. The heel point were the sailboat will sail faster.