My personal experience on my Nacra 6.0na, is that the jib downhaul does work. It makes a noticible difference on that boat anyway. On other boats it might not make as big a difference. I also compared an old jib to a new, flatter jib, and much preferred the old jib in light/med air.

I would be curious to know if anyone else is doing anything with this control, and how they trim it.