My 2cents...

Don't underestimate the gains to be achieved through stiffness! I think Wouter's earlier estimates of a minute over an hour are probably too conservative by far!

My reasoning: On my windsurfer, I have two 30cm fins which are almost identical in chord, profile and plan, but one is noticeably stiffer (I can feel it flex maybe 5mm when I push with all my strength, whilst the other feels like a brick!). That might not seem much, but it's enough to get me planing in almost 3 knots of wind speed earlier (11 as opposed to 14 knots) OR get me going in 15 knots with a sail that is almost 1 sqm smaller (6.0sqm vs 6.9sqm).

That is a HUGE difference from one small fin. These results are consistent over the 4 years I had both fins, always using the same anemometer and board. Sure, fins aren't the same as hulls/platform, but you get my point.