Nav wrote:
“The starting point when dimensioning the rig is to calculate the
righting moment. It is commonly agreed that a heel angle of 30o is a
good design angle. This corresponds to a reasonably high wind strength
with the sails still generating high loads and the boat making good
speed through the water. Letting the boat heel over more....in reality
means a slower boat owing to increased resistance, with a
correspondingly smaller dynamic force.”

Principles of Yacht Design Larsson & Eliasson Adlard Coles 1994


Funny, you picked almost the exact same words I would have

"Letting the boat heel over more....in reality means a slower boat owing
to increased resistance, with a correspondingly smaller dynamic force.”"

They're talking about stressing the rig, not best sailing efficiency.

DSK