Hmmmm.... very interesting!
I wonder how you are measuring the wind speed.
30-40 knots is usually 85 liter boards (even for sailors much heavier than your 56 kg.) and 4.2 m2 or sails.
If you are indeed using a 5.5 m2 sail in more than 20-22 knots, I can see why you are having some issues. Did you use these size sails on the Rio M?
When you get on smaller boards (around 100 liters and less) you will find you don't need a 5.5 m2 for 20 knots+ and you for sure don't need a 4.7 m2 for 28 knots or more.
You'd be pretty much blown off the water with sails that large.
Sorry, something isn't clicking into place here.
If you are 56 kg. for 20 knots you might g0 4.8-5.2 m2 but you don't need anything larger than this.
For 25 knots 4.2-4.7 m2 should be good.
For a solid 30 knots, 3.8-4.2 m2 would be about right
For 40 knots, 2.8-3.5 m2 would be about all I could ever handle and I weigh 80 Kg.
Hope this helps,