Its not (just) to do with the boards.
When sailing really deep (almost on a run) our normal sized "freeride" rigs just dont give sufficient power to maintain board on plane.Once you drop off plane you are lost.
Put an 11 metre race sail on say a Carve 145 and it will sail loads deeper (on plane)
Its sort of like spinnaker on dinghies.They can sail (plane) loads deeper when using chute of some sort.
Stick a normal 6 metre freeride sail on a Formula and I dont think it would sail any deeper than a big freeride design.