Hi all. Dejavu I also own a Flydoor XL 2012 that, you know, has only three fins to the upwind edge. The reason is simple: it is possible to ride the board almost flat so other two fins don't add much in terms of stability and upwind performances. On the contrary, they only add weight and drag. Nevertheless with only three fins it remains very stable even when I ride toeside. IMO I don't think adding two more fins leads to a substantial improvement in performance. On the other hand, due to the less width the little sister (Flydoor L) needs them in order to improve the toe side control and, why not? The stability during blind riding, inverted 540 and so on.
Oooops I was almost forgetting, here's a video concerning some sessions I had with the Flydoor XL:http://vimeo.com/23947387
...just to clarify some concepts