Hi and thanks for your question because I'm sure there are others thinking the same thing.. It's great that you're showing an interest in downwinders because IMO it's the best thing about stand up paddling.. So much fun.. So much to learn.. and so good for your fitness..
I don't mean to be rude or put you off the whole idea.. but.. asking about joining in on a downwinder on any type of wave board or anything smaller than a 12'6" flat water board is a bit like asking to join in on a long'ish road ride with a bunch of roadies (racing bicycles/triathletes etc) and saying that you only have BMX bike but would it be ok to join in on the ride... For sure its do'able.. But not only hard work.. not much fun.. but a little bit NQR..
There are some people that can do it.. (DW a wave board).. But it depends on the person.. skill and fitness level.. the conditions.. and the type of wave board.. (some work.. some don't) .. But.. In saying that we all started on big wave boards just like yours.. This was before there were flat water boards or race boards.. and we had fun.. Heaps of fun.. But if you asked me now to do a downwinder on my old 11'6" Nalu now I'd probably say no.. Too slow.. too much hard work.. and the board shape is one of those wave board shapes that don't really work.. If the nose goes under (like it will) the board will stop and you will fall off.. Again and again..
You are still more than welcome to join us tomorrow.. The forcast has changed to only 15-20 knots but the weather can be sooooo unpredictable.. We might get 10 knots.. We might even get 30 knots ( fingers crossed

) I'm happy to talk to you more and offer you as much advise as I can.. The best advise now regarding your main question is to pick a seabreeze day.. Like we get all the time in summer.. Go down to St Kilda marina.. Paddle off the end of the breakwater wall at the end and you will see wind swells going past the end and heading towards the shore (about four hundred meters away) .. Paddle hard and try and catch them.. btw.. You can also do this off other places like Sandy marina.. There is an art and skill in doing this... It's nothing like catching a wave in the surf.. Once you get near the shore.. Paddle across towards the marina where there's a wind shadow and flat water.. Head back out to the end of the breakwater wall and do it again.. I often do this in summer.. It's great fun and great practice for doing downwinders.
Hope we see you down there tomorrow anyway..

(hope I haven't put you off)
Here's the new forecast..
Saturday 4 October
WindsNortheasterly below 10 knots tending northerly 15 to 20 knots before dawn.
SeasBelow 1 metre, increasing to around 1 metre during the morning.