Chemical industry or fluid mechanist are able to express an opinion on this question of performance.

"In results done by Otago Flume Laboratory they found that its satin finish gave a 15% less drag coefficient that a high gloss surface. "

May be it's only marketing, like the following quotation from a recent F18 flyer concerning a point about paint performance:

"saving 3 kg per hull compared to gelcoat allows to increase the laminate and internal structure of the hulls in order to improve both stiffness and durability."

Here the key point is the fair competition between boat builders. That's why rules can change with one year delay notice.

At least there is a doubt. TC people did not find consensus. Here I do admit in the doubt the World Council choose a conservative way for the clarification (no extra rules).
Way which is just confirmed by ISAF.