I have to say that Lessacher's Duo carbon weed fins are mighty powerful for their size, and I think it's attributable to their asymmetrical foiling and stiffness. I have a 28S and a 34, and I'm very impressed with both of them. I use the 28S up to a 7.1 sail, and there is absolutely no crabbing or spinout problems. However, I'm using it on a custom freeride board that's about 100 liters. I use the 34 on a bigger course slalom board (a 1998 custom of about 120 liters) for my 8.3 sail, and I have to be frank that it gives comparable lift and drive as a 20" TA SB Weed Blade that I used to own for use on the same board.
Another thing that's notably different with the Duo Weeds is that they aren't set forward of the finbox like so many of the weed fins on the market.