Tested today the Serenity with NP RX2 10.6 sail in slowly increasing 4-8 knots of wind (over 1 hr) with 70 cm fin, and in slowly decreasing 8-4 knots of wind (over 1 hr) with 40 cm fin. I'm 183 cm tall and weight 65 kg.
IMHO, the board does plane in the sense that when the hull just below the carry handle climbs over the bow wave, half of the hull is out and above the water surface when being overpowered in...8 knots of wind. This is starting to be a bit tipsy and scary, especially when the large 70 cm fin wants to overturn the board upside down. The shorter Shallow 41 cm fin does increase the control in overpowered mode, however.
In 4 knots of wind, it does not plane, the first 25% of the nose (around the feet of the painted Tiki on the deck) is out of water when the hull is gliding silently (with only a sssssss sound) through water. The most surprise is the silence made by the hull when sailing in such low wind, hence its aptly name Serenity.
For me, a great ride from 4 up to 8 knots. At 7-8 knots, I keep the same 10.6 sail and just switch for a planing board. Not tested yet below 4 knots of wind but I guess I would need enough low wind to still be able to steer the board. More tests later (e.g., with smaller sails and fins in higher wind).
test with 70 cm fin :
test with 41 cm fin