46cm fin in isonic 101 2010

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I'm experimenting with an old fin's interantional slalom fin in 46cm in my isonic 101 for extremely low winds and 8.5 sails. At the very low end, the fin planes like crazy and goes upwind easily. However, I hit a wall very fast when the wind picks up even a little.
The fin is a very stiff carbon fin that dates back to the mid 90s. When I bought it at the time, it was supposed to be top of the line and quite expensive.
Is the fin a mismatch for the isonic or is it just junk period? I'm considering trying a smaller 44 cm fin for ultra light wind planing with the isonic Isonic recommends 42 as the biggest fin.