Someone had to of tested this on a race boat. Unless it's illegal? Anything that would make a race boat run faster, and is legal, must have been tested. Right?
On RC boats, the turn fin on a deep v is set so that at top speed it does not touch the water. When turning, as the hull lays on its side to turn, the turn fin is in the water. Would the same principal apply? I would have to say yes.
We also use only one fin on the inside, (turn side), of the hull.
For a non-race boat they'd have to be on both sides. Same with RC.