Yes, "Formula can outpoint Slalom", but your question related to upwind VMG. And yes, on a upwind leg, Formula will have a better VMG than a slalom board, assuming the leg is directly upwind.
My above comments only related to Davide's comment that his CA was faster downwind than a formula board and I stated that it wasn't so if the mark was dead downwind.
I doubt that a slalom board can achieve 40 knots in a 20 knot wind in open water with the likely chop and swells. If "approach 40 knots" means 35 knots, I might buy it.
Fun discussion regardless.