Full disclosure, I got sone discount on my sonic 1's and sonic 2s, but they are still the most expensive kites I have ever purchased (previously rode chrono, dynos, edges, rebels)
Ibe been riding the 9, 11, and 15 sonic 1's since August 2015 and have been happy with them.
I got my new 15m sonic 2 last week just in time for a race this weekend. Also borrowed a 13m sonic 2 for the weekend. Haven't tried the smaller sizes yet.
Biggest changes from sonic 1:
-more solid in the air. No matter how loose the lines get, it just wants to stay fully inflated.
-the lotus material really sheds water. Trough user error I crashed the kite twice over three days of racing
and dipped the wingtip while looping through several jibes (still getting used to short lines) where I would have expected lots of water coming off the kite after relaunch, barely any water went up with the kite. Relaunch seemed really good. During one of the crashes it was pretty light.
-No backstalling. To go fast downwind on the sonic 1 I had to constantly play the bar to let it breathe. With the sonic 2 I just let the depose all the way out, and pulled the bar to the chicken loop, and followed it downwind. This led to more consistent speeds downwind and lower angles.
-At first I thought the turning speed was slower, but I realized with the sonic 1 it would easily backstall and pivot in place. The sonic 2 just keeps moving forward steadily through the turn. Was not a problem after a bit of adjusting my jibe process. Once I adjusted my starting point for the loop a bit higher, it was a lot easier because the power delivery was smooth through the loop.
Didn't get a chance to try back to back with the sonic 1, but I'll post back when I get a chance.
The only other foil kites I've flown were the chrono v1, speed 4 and speed 5, so can't comment vs diablos or r1s. Based on my results relative to other riders I race with regularly, I think sonic 2 is competitive with the r1v2 and Diablo 2 for less cost. We didn't really get down to survival light wind conditions though, so will be interesting to see how it compares to diablo 2 and r1 v2 which I think are quite a bit lighter.