RE: Ian/Jesper/Remi - Isonic 115 & Fins
In my experience I would not go much bigger fin than 42cm for the Isonic 115. If you go with a 44-46 fin it don't really make you go a lot better upwind. The balance will be a bit off. A good 40-42 fin will do the job.
Good quality fins is more the key.
I use the Vector Fins Maui the model called Canfire. It works really well on all the Isonic models, and the finish on the fins is always perfect. They have good power, so I recon a 40 cm Canfire would be enough for your Isonic 115 with the 8.0. (This with your weight in mind as well)
On the Isonic 122 I use the 44 Vector Canfire and this is my biggest fin for light wind (upwind and downwind)
I know Pawel (POL 11) used his 42 Canfire on a 135 Isonic with good success as well in the PWA slalom for light wind.