Hello Ale,
you are right. They do turn upside down very quickly. Compared to the Dart 18, which doesn't invert readily, the Sprint 15 has not as much natural buoyancy in the mast.
About half the fleet at my own club put foam in the top section of the sail. It does not appear to effect performance and is usually good enough to prevent the boat going turtle as long as you don't have holes in the mast.
There are details here:
https://www.shanklinsailingclub.com/con ... floats.php
Any sail maker should be able to do this for you.