RE: Improve performance with new fin?
I am sure you can get some preformance enhancement from a new fin. The Lessacher is a very good fin, but they sail VERY big (lots of power). The 42 cm Slalom duo is probably equivalent to something like a 46-48 cm Curtis SR-6b. This sounds very big for your C122 unless the only criteria is marginal planning.
A typical slalom fin (like the SR-6b) in 40-42 cm range might make sense with your 7.5, but that would probably translate to a Lessacher 36 cm (really hard to say exactly...)