Hello,
Abit late reply maybe but here a link:
www.facebook.com/wytze.hoekstra/videos/2535190069836546/I think it is public.
But I am using the 137 with the NP alu foil for about 2 seasons. Maybe 20 days of foiling.
Just used a thickplastic cover and large rings on top. I havent strengthend the box yet :).
It is not a perfect setup with 8.6 and 7.0 but atleast you can give it a go and see if you like foiling.
It depends alot on the foil you will be using. Maybe race foil will give abit more balans.
At the moment I still have quit some backfoot pressure but it's ok.
With 5.3 you notice the sail is alot lighter and you can start foiling directly on the foil.
With the larger sails you need first planning speed before you foil.
From what I have heard the mistral 137 2014 has a stronger box than the 2013 model.