Unfortunately due to our small population over here we don't have as great an option for boards as you have on the east coast. When I was looking for a downwind board there was only two I could find to demo they were the STB Pin or head down to Margs and try the JL. Both of these were 12'6 and there was nothing larger over here that was suitable for our conditions.
Realistically our downwinders are a WSW swell with S, SSE or SSW breeze. This means you tend to get slapped around a fair bit by cross chop. Having Rotto offshore is great but I am unsure if you would be allowed by the authorities to paddle back without a back up boat as you spend a fair distance in the shipping lanes. (and we are the Nanny state after all.)
As it is at the moment the group I paddle with are on 11"2s and I'm on a 12' Glide and the difference in speed is very noticable. I would only see them at the start and at the finish if I was to step up to a 17. Sure I would love to give the bigger board a run and no doubt I would enjoy it, but why step up when theres no-one to push or compete against. Half the funs paddling together talking s&%t, hooting and generally forgetting about whats back on the beach.
As the sport evolves over here and its happening real quick we will start to step up in board sizes. Also at the moment the three comps that have been run so far are mainly aimed at the 12'6 sized boards when it comes to prizes. Why be a hare when the tortise is going to get the prize???
At the moment its hard enough to get guys to try downwinders without scaring them off by thinking they need to outlay more cash for a larger board to be competitive.
No doubt it will happen over here(14',17's) but its going to take a lot longer as its going to take a while for second hand boards to filter through, as we dont have the same sized market as you do over there.