Gorgo said..
A wider board will have better upwind performance because it maximises the planing area under your feet. Wide boards are less susceptible to flexing than longer boards, which reduces rocker. A stiff board will also go upwind better. A long, stiff board also has more speed and upwind performance. A long, flex board will be slow.
I thought I'd read somewhere (most likely on here) that a flexible board allows you to flatten out the back foot (where the weight is) and knife the edge more on the front foot. When really focusing on upwind I try to do that, but the gains are marginal (<3 degrees). That's a gain over the same flexible board though, so I'd be interested if it counteracts ALL the rocker issues.
I've also found that widening my stance made a couple of degrees difference, but couldn't really nail down why (either able to twist the board more, limit the flex in front of front foot, limit flex behind back foot, create more torque on the board to engage fins more, etc.). Only evidence from one board with 3 different stance types, but measurable on the GPS track.