The Sonntag brand is not one of the biggest for Slalom and Speed fins out there but they definetly produce some quality carbon fins.
The SL-R fin in standard stiffness is already a really soft fin that generates a lot of lift. The crease in the middle section allows the fin to create more lift from the lower part of the fin.
From my experience the fin gives a lot of power in all conditions, especially in the medium wind range when going half-wind or slightly downwind. I experienced an increased average speed of ~1knot compared to my Select S1 fin. For this reason I like to use the SL-R especially for downwind slalom courses, that are usually not so deep downwind. In tough speed conditions the fins do their job well, but are sometimes a bit difficult to control (at least for me ;)).
I currently own 3 fins of this type: 49/46/39; all in standard stiffness and rake (you can order the fins in any stiffness and rake you like). The 49 I use only on my 85 Manta (138l), the 46 with my 81 Manta (129l) and the 39 with the 71 Manta (113l). I love the 46 and 49 as they really gave my speed on the slalom course a boost, but somehow I cannot do the same trick with the 39. Maybe I need to test a bit more or need to get the smaller fins in a different rake/stiffness? For now at least I am very content with the larger sizes.
As for the quality of the fins, they seem to be produced in a very high standard. I have not had any problems like cracks, osmosis or anything alike. Of course it’s a carbon fin so you have to take care of the fin especially to the tip and the thin outline as those parts are easily damaged at impact.