Agree with previous posts : take a bigger sail to improve your low end in light wind.
However, I'm not sure it's a good idea to think about an UltraSonic with your current NP V6 6.0 m2 sail. This is a bad match mainly because the sail is too small for the board or the board is too big in high wind with your 72 kg weight. Is it doable ? Yes. Is it fun ? No, not really.
If you really want an UltraSonic, then I would choose to pair it with say a Severne NCX 7.0 + 9.0 m2 no cam sail quiver or with a Severne Overdrive 7.8 + 9.5 m2 3 cams sail quiver.
The low wind range limiting factor of the UltraSonic is not the board nor the fin, it's the sail size. Reason to have big sail in light wind is because the UltraSonic is a big board with a big fin and it needs quite a big sail to overcome the drag in light wind. If you take a sail that is a bit too small, the gain in early planing will not be that spectacular, especially with 8-10 knots of wind : you will schlogg in 8 knots lulls and plane in 10 knots puffs with say an 8.5 m2 sail, not really fun to do... If you want to lower the planing threshold later on, you can do so by adding another bigger sail (say Reflex 10.7 or Overdive 11.0 m2) + 62 cm fin. You will have plenty of room to improve your light wind skills step by step should the need arises later on.
Last edited by Jean-Marc; 13th March 2013 at 10:47 AM.