Well, with 30 knots (7 Bft), I wouldn't like to be with a 5.4m2 sail in on-shore conditions, with waves coming up to you ready to launch you several meters high. Off-shore, yes of course, that's a whole different thing.
I also heard that vent d'est is more stable than Mistral, so perhaps I can't really compare with the conditions I had over there. Anyway, I also felt from time to time underpowered with a 3.7m2 to get into planing (you always have big lulls in these kind of high wind days), but once planing it was no problem at all. Only difficult to go through to the shore break, but like I told, swimming far enough behind the shore break did the trick. I would not advise unexperienced windsurfers to use too big sails in heavy on-shore conditions. Enrico is of course heavier than me, so 4.5m2 would be about right. Just my opinion.