Isn't there two sides to this discussion?
1. Racing and going fast.
2. General leisure and exercise paddling.
I can't comment on what is required for racing, what is fastest and performs best, because I am not into that side of the activity.
However, I can comment on general paddling both in the ocean and in still water.
Obviously waterline length is a function of glide and speed as is the strength and ability of the paddler.
Not all paddlers are competitive (yes that is correct) many paddle for the exercise, as an adjunct to their surfing and to be on the ocean.
I paddle a 12'6" board because:-
1. I can carry it more easliy.
2. I can put it on my carry more easily.
3. I can store it more easily.
4. It does pretty much what I want it to do.
5. it gives me a good workout.
6. I smile when I am paddling it at sea.
7. I have no intention of going way out to sea in heavy winds.
Your board chioce (in my opinion) is a function of what you can afford and what you want to do with it.
Cheers. ET.