Zusspa,
I am sorry to tell it, but believing that you need carbon fins for the depth is a myth. Personally I do not dive too deep, but doing 30m with stupid cheap plastic fins poses me absolutely no problems. And for example Dave Mullins (multiple WR holder and a guy who can dive below 100m) tested many models of carbon and plastic fins, but his personal preference are plastic Cressi Garas - he chose them over C4 or other high-end models. And believing that you need stiff fins for deep diving is another myth. What you really need is improving your kicking style, and being fluid - you`ll find that you`ll make much better performances with them than with stiff fins that are in most cases inefficient and just wasting your energy, causing cramps, and muscle fatigue. That`s not only my personal opinion, but also Dave Mullins and other top freedivers often voice this opinion, Martin Stepanek dove to almost 90 meters with small rubber fins, which are everything but stiff.
I know Beuchat carbons too, and have tested them in the pool. I like Beuchats, and use their plastic fins, but the carbon model did not awake any excitement in me for them. In fact I prefer the plastic blades (cost 23€), and have no problems making nice distances with them (125m DYN today, for example). I had carbon C4 Flaps too, and although I liked them a lot, after I broke a blade, I decided not to spend money on such expensive equipment before there is no other way to improve. I came back to the cheap Beuchat blades, and doubled my PB (Personal Best) since then. So in no way I regret my decision.
However, if you really look for good carbon fins, I would chose soft C4 blades over those of Beuchat anytime. Cannot speak for Picasso, because I never saw them, but I tested many other fins, and if I could get some for free, I`d choose the C4. But as I wrote, I am not ready to buy any such fins anytime soon - the plastic is good more than enough for improving, and even for deep diving.
Just my 2 cents, but of course, you`ll hear opposite opinions too