I admit I know nothing about downwinding but my limited knowledge is sufficient to say that surfing style sups really are not at all suitable/fun for a downwind run. The longer sups are great for what they were intended - that is, flatwater and small surf and a bit of a slow coastal cruise. However there are so many offshore/downwind boards around at the moment and many very good value as new second handers coming through all the time so if you wanna go offshore any distance then get a board made for the purpose. Another really good option is the AST construction in the Naish boards which gives you a cheaper option in a proven performer and may allow you to get into a new purpose built sup.
Its made for BOP style races so in and out through the surf and it will also be a great training board. I will race one in a couple of weeks and do up a review on it.
Jacko