An interesting aspect here is that due to the rules of PWA slalom 42 racing, the pros are still racing on 08 boards. But 09 boards have of course gone through some rigourous testing and it is safe to assume that the fin(s) provided with the reflect the "best size in mean". But afaik, the fins have changed a bit between the years so a bigger oem size will not necessarily mean the board in question require a bigger fin in a given set of conditions.
Many of the pros also use fins from other sponsors and they may work differently. Also rider style will vary. So in the end we will not really know if the pro racers will change their fin sizes until real racing on 2009 boards begin - or at least not until the 08 season ends and training on 09 boards start.
But Remi and other developers as well as the team racers like Taty will certainly be able to give a very educated guess.