I had a bit of a back to back comparison Monday beteween a C3 Venom 42 and a KA Slalom 37 . Not really apples for apples due to the size difference but interesting anyway.
Started on the C3. Was on a 69 wide Slalm Board and 7.1 Race sail. The fin felt great, lots of power and rock solid. Was having alot of fun on it. The wind picked up a bit and it was starting to lift the board a bit much on the flat water at the PiT so I felt I could change down,
Grabbed the KA 37 ( and naturally the wind backed off a bit) The KA was noticebly different in feel maybe partly due to the smaller size. The board instantly felt much looser. I found the C3 quite "stiff", made the board feel on rails a bit where as the on the KA I felt more manouverable. It felt more slippery but I could not load my back foot as hard(37 small for this board unless fully powered up). I thought it would not be as good upwind as the Venom but it was , could crank lots of height out of it.
Speeds , well it was a bit up and down due to the wind gusts but recorded pretty much identical speeds on both.

So both fabulous fins. I feel the Venom gives the board a stiffer more directional feel than the KA with the KA feeling more slippery and the board a bit livelier with this fin in, they do have a different character. I have done 37 knots on the KA in this board on another day with more wind, not sure I could do that on the 42 Venom I think it would be too lifty.

Good fun and great fins.

Also consider (well... for future reference) the Vector Fins 'Canefire'. I have a 42cm for my 111-Isonic for my 8.3 CR and it gives me insane lift and stability.

A friend of mine tried it out last week when sailing in SW France and he concluded that it gave him as much lift as his Select 47cm slalom fin.

I purchased mine directly from their website and had them in my mail box one week later (Maui to Europe).