Scotty, I wish it was me on the video !!!
That guy is Benjamin Tillier, used to be part of the french best rsx/formula windsurfers.
He's a sport physio and works at developing foils for various brands.
Holding the uphaul is a bit of a trend here in New Cal when you want to go upwind in light wind. Some guys are now even doing it across wind to keep the sail upright and build more speed.
I "think" the maui he's using is a 6.0.
the ncx he overtakes looks pretty big, 7.9(?) and looking at the water on the spot probably around 12/14 knots
(I sent all the questions to Benjamin to have more accurate answers....)
Your 8.4 on a foil would get you going in 6/7 knots !!
My first attempt last week got me planning in 12 knots with a no cam 6.0 sail. (95 kg on a xfire 122)
From the feed back I got from Benjamin, the board only needs to get you to semi planning, after that the foil does all the work, so I suppose the 107 should do the trick !