I know where you are coming from. I too went straight from a full on modded 92 650sx to a dead stock 96 rn sj and wondered what all the hype was about. At the time I could flick my 650 around like it was not there, fly it off of waves and get some pretty sick air, the ski felt like it handled the surf with confidence...the honest truth is for those of us that started out with a 650sx learned how to make the ski work with us...as long as you mod the crap out of it...you made it a part of you and learned to love it. I compared my heavily modded 650sx to my buddy's old dead stock 650 and the difference was night and day. I still have my 650 and I still love it but it will take you about a half a season at least of frequent riding (2-4 times a week) to get a good feel for your sj.
There are a few little things you should do though to it to really help change your impression to a much better one. First up is drill and tap a new hole in your pump nozzle so that you can mount the steering cable ball as close to the nozzle as you can. You will have to play with the adjustment on the cable ends to get your steering throw to balance out in both directions, and the second combination that makes all the difference on a totally stock sj, and is all you really need for your first season is a UMI quick steer turn plate with zero degree bars and a set of ODI lock on grips. Finally, which can be put off but after having them I would never go without them again is footholds. It is a bit of work...messy work at times...but so very nice to have.
Because my Yamaha experience and knowledge is limited since this is my first Yamaha ski, I am not sure what year they ditched the aluminum impellers and went to the 13/18 stainless. But if your ski has the aluminum impeller and if you have the finances, time and patience, the next two little mods I personally would strongly recommend is a new stainless impeller and top loader intake grate. You can decide on impeller pitch based on your riding style, I prefer holeshot because speed to me is boring, and from my limited experience with various impellers I have found that without going to a big hub impeller a Skat-Trak 10/18 is an excellent choice. It gives you a good balance of both worlds but is very noticeable on hook-up over any stock aluminum impeller. A good temporary intake grate mod you can do to your stock grate if money is an issue, just cut the center rung out of the grate and run it that way. With the 650 I lived by the free mods method until there were no more freebies that could be done...some of these mods can apply to both makes such as this one.
Once you do the handling improvements, you will really come to appreciate your ski and have a lot more fun than you thought. You will find that even with a quick turn plate on the Kawi, the sj will well out carve the Kawi every time after you have done the steering improvements. You will actually start wishing the Kawi had a lot more steering throw after that, I know I do lol. Plus the tray width will be one of the most un-noticed but most remembered when you spend enough time on the sj and then jump back onto the Kawi tray. The Kawi just feels cramped and restrictive when your feet are so close together and you can't steer anywhere near as impressively as the Yami. Just give it a half a season and then go for a run with the Kawi for a change, you will find you are much more at home and confident in the SJ tray.