For twintip, for that size and your style in the howling winds you are considering, you should be interested to compare the Speed FR 18 instead of Sonic 2 with Speed 5. More direct feel, more kick in the butt, and more durable. IMO, Sonic2 beats FR only in light winds, mainly for its stability and weight (less tip collapse).
Regarding the Speed 5, forget it. All 3 generations of Sonics are better for hang time, wind range, turning speed and even relaunchability. But I'm sure you know this.
Momo 13 comment in another thread: Having both Sonic1 11m and Sonic2 9m and using them both for freerace hydrofoiling and freeride/big air on TT, I also find the Sonic1 better for TT for boosting. Good point of Sonic2 is that you have less tip tuck and colapses, so you can avoid droping your kite in the water, but weak point of Sonic2 is that its turning radius is larger and it keeps more power during a abrupt turn, so it's harder to do a nice boost, I tend to feel overpowered quickely.
Hi Toby, I think that the Sonic 2 18, compared to the Speed 5 of the same size is 10/15 per cent better in terms of hang time and lift. The wind conditions of my home spot are usually marginal and I must admit that the Sonic 2 18 is the best kite that I've ever had for low wind airstyle tricks.