My weight is 90 kg, to start planning at 8knots with regular freeride board and 8.5mē sail is impossible for me. I need at least 13 to 14 knots wind to start planning with this combination, perhaps with Severne Glide or NP Helium I could start planning little bit sooner but I would still need more than 10knots wind.
Specialized light wind sails like Glide or Helium have more low end power but they are harder to control in gusty winds. Normally at my windsurfing spots wind is from 7 to 20 knots with average 12 knots. With 8.5 mē NP Helium and freeride board you are totally overpowered at 20 knots, but you can still survive with formula.
Generally surf board with wide tail starts planning sooner. Usually slalom boards have wider tail than freeride boards, for example Isonic is wider than Futura and it starts planning sooner with same sail.