Jean-Luc I think there are at least three things to consider:
Ease of use: I have a crossover 11' SUP and tried a race 12'5" SUP. I did not like it. Yes I was going faster but I had to pay much more attention to what I was doing, so forget it. I SUP to relax. Same, as you point out, for a beginner wind-sup: they really want something nice and stable and easy.
Diminishing gains: yes a long serenity shaped board will be more efficient and faster in displacement, but do we care? It is displacement sailing, and it is fun and playful, but what if I go 2 knots slower than another displacement board? Is that important?
Length: I don't think one can underrate it. I can put my 11' in my van, I will not buy anything longer. And carrying a 10-11' board on top of a car is quite different than carrying a 12-13' (14-15' yikes!). I am with BelSkorpio here, one of the main reason for the popularity of kiting is the fact that you can carry it in a bag.
So ... it would be great if starboard offered a serenity "on order" but it is probably a niche product. Probably making it shorter would not change the equation much since the few people that would buy it are in for the efficiency and don't care about length.
Last edited by davide; 11th April 2013 at 02:27 PM.