Nive vids you have there (http://contour.com/videos/watch/west-kirby-14th-may
). I was surprised to watch how good you can come back upwind to the starting point in just 1 upwind leg.
I tried once a ReflexIII 8.6 sail on the iSonic W53 speed gun just to see how it would (mis)match. Surprisingly, it went fairly smoothly and I was still under full control while being frankly overpowered in the bigger gusts (the iSonic 117 Wide was starting to be at the upper limit in 16+ knots gusts for my 65 kg light weight). The Deboichet SL4 32 cm fin handled that big sail without any problem at all. Need to move the boom higher up because the 32" harness lines are a bit long on this narrow gunny board (it's perfect on the 117 Wide, however). So, I surmise that the W58 option would be spot on for a heavy weight rider.