(from my perspective on that 1 shot) looks like your leaning too much towards the back of your board pulling your sail too far back killing power. Front hand is too far forward too (this alone costs speed). I think your lines are too (futher back, you can handle/get more wind).
Not sure about ideal angle for top speeds, never measured it, I guess if straight across the wind is 0 and straight down 90 then it would somewhere around 25-40° (never measured it, but when I fly downwind I really lose a lot of height).
You say the shot was made at a somewhat unlucky time, otherwise I would also have commented that the board could be more leveled out from front to back. Your knees are bent and I think you are puling your front foot up (something you do when going upwind). Your front leg would normally be straight nudging the board downwind (if it doesn't feel like on the verge of catapulting it just aint right).
Your arms are bent, lines long enough/commit enough (soley) to your harness?
note to the "hunched stance":
If you don't have much wind (or you're trying to get the very last out of your sail just before catapulting) you crouch OVER the board keeping the sail UPRIGHT and PULLING (keeps board flat (minimal tail drag) and the power on).