The SpeedSlalom fins are unlike the name suggests real Slalom fins. I use the SS in default stiffness, which is M-. The outline is a bit smaller from the middle section to the tip, resulting in a more twist in these sections. I found this fin to work particular well in chop. With the smaller outline I need about 2cm more length compared to for example the SR-4.
I currently have a SS M- in 42cm in my quiver, that I use with a 81cm Tabou Manta and my 7,8 Severne Reflex 4. Being a bit underfinned with this sometimes the speeds are really good and I did 33,7 knots topspeed so far.
I can recommend this fin for getting great speeds on you slalomkit, especially in conditions we get at inland lakes with gusty wind and small chop, due to the fin accelarating quickly. The price of that fin should be around 200€new/140€ used making it an affordable slalom fin as well.