I come from a background of riding kawis and bought an 09 about a month ago. To the OP whoever told you that the 08+ was a stable ski compared to a 650sx really needs his head examined or more tray time. My 650sx is a tank and its a very lazy, but easy carving hull. My 09 SJ is at least as unstable as my 550, but I love it. It does its little wobble in a straight line and you do need to get used to it. As others have said I think its a combination of lots of the hull mods working together, specifically water hitting the edges of the built in tubbies which aren't always in the water. If you want a go straight line smooth ski, a 96-07 would be a much better choice. But all superjets I've been on exhibit "chine walk", from the hull bottom or tubbies, they all wobble. Ride plate and intake can solve a lot of it.
For me the 08+ hull feels pretty natural after riding kawasaki 750s a lot. I find when I'm going on longer rides I really wear myself out because I'm rarely going straight on the ski, I'm constantly turning and carving. When you are leaning and carving I find this ski is actually quite stable. It might rock a little bit side to side going straight normally, and even more under deceleration, but if you take even a high speed turn leaned over, it will just grip and go. Very smooth even if you're going through chop. This hull is a race hull or at the very least a hull meant for someone who really wants to get something out of the ski. It requires rider involvement more than any ski I've been on, which is is why I spend so much time on mine.
I spent 10 minutes riding a 2014 SJ last year before I decided I wanted one. I am so glad I made the purchase. If you have fun leaning over and carving, but want a more playful ski than a sxr, then this is the ski for you. The lack of stability you'll get used to. I just can't get over how much more advanced it is over all the 750 skis I've been on. I don't feel like the hull is hampering my riding abilities. And again back to the OP, if you're coming from other more stable skis this might require some getting used to, but wow what progress. And if you don't enjoy it there's always plenty of options for swapping down to a 96-07. Those hulls are really much more stable.
One more thing, I'd recommend always riding a ski before you buy one. Everyone's tastes and preferences are different. And also there's plenty of us on here who'd be willing to let you try a certain type of ski.
