I have often wondered what happens to the little darlings when I fang through a load of jellies on Lucky Bay. Maybe they just keep doubling up. The only time they cause me any grief however is when I get bit of one on the deck during a tack or gybe. Then it isn't the fin spinning out

I use a 32cm fin with my 105ltr Screamer down to 5.25m sails and I am your weight. Very rarely do I have spinout unless I am careless in the chop, or thump it at low power. Powering up the fin requires you to push against it with a steady pressure, so if you are powered up it should not cause you grief.
Maybe you are not as powered up as you think. 5.4m on Lucky Bay in recent weeks seems a little small apart from rare moments. I am normally out on a 7m and I am normally the small one around our group.
Anyway, hope this all provides a little more input for you and hope to see you down there when the wind gods are in a good mood.