Poster #64 nailed it on the head.
Slalom boards will never beat a formula board upwind, in any conditions (1 knot to 40 knots), assuming both boards had the appropriate sail size. In sub-planing conditions (I have raced formula like this a few time in local club races) formula will slog upwind better than any slalom board (70 cm fin & volume makes the difference).
Formula boards will plane quicker than slalom boards so once there is 6-8 knots of wind, a formula board will plane with a 12.5 m sail and can point a little upwind. A slalom board will still be slogging. At about 10 knots, both could be planing, but the formula will point significantly higher with as much or more speed. As the wind picks up, the slalom board may close the difference a bit, but it will never match the formula board. As poster 64 says -
"I've never found sailing upwind to be a problem for FW gear -even in totally overpowered conditions. That's not the case for more narrow tailed boards which tend to bog down, spin out or blow up when sailing way overpowered upwind."
Top formula sailors can handle 9 & 10 meter sails in 30 knots (upwind and downwind). Put them on a 6.5 or 7.5 and they probably could go up to 35+ knots if the water was somewhat protected without huge chop. Upwind doesn't matter much, but running downwind can be pretty wild in rough water in 30 knots of wind.