I think you will find on a board as heavy as the Rio that the 54 cm fin will work nicely with all of your sails.
I've sailed the 2008 Rio M (the next size larger) with up to a 60 cm fin and there was
no problem with a 7.5 m2 rig on fresh water.
I sailed the Rio M yesterday with a 39 cm True Ames shallow water and had a little bit of spin out well powered on a 7.5 m2 Sailworks Retro.
So, I'm pretty sure your new fion is going to be much better that the stock fin.
Hope this helps,