Hi Harrow,
I bought a 37 cm Select Pro-Am fin today, and it made a huge difference compared to the 31 cm freeride (I think it's dakine?) fin I had on before.
Earlier planing, and better pointing...
I now feel as if my board is 'in the window', so a small change in anything is noticeable (harness line posiiton and length). Before this, a small change wasn't noticeable because the small fin was drowning out the other effects.
I even found myself planing when my feet couldn't possibly be near the footstraps... a real first for me, normally I'm the last guy on the plane

ymmv, but that's what my experience is.