7 knots? Can't be right unless all slalom racers are under 50 kg. As a formula sailor at 80kg, I feel confident that it's 11 knots for slalom.
With an 11.0 and a F160, it takes at least 8-9 knots for me to get going with a lot of pumping.
Slalom boards of equal volume will not plane quicker than Formula boards.