I have a 2004 SJ all stock except 0* bars and bilge. I frequently run bouy's and I would like to believe I am fairly good at it. I probably wouldn't win any races, just making the point that I can handle it pretty well. My SJ has great handling in the flat and I can hit the corners at almost WOT and power through them.
However the second other people get on the course and the chop picks up, it's a whole different story. The SJ pops out of the water too much, comes off the wave and bashes into the next. I spend a lot of time with the pump out of the water on the straights. It doesn't matter how I move my weight, it's not porposing, it's airborne. The same happens in the corners where the ski is digging in with a lot of G, but comes off the chop and bashes into the next wave. I can almost hang with a stock SXR on flat water, but the moment the chop picks up, the SXR is gone and I'm either too exhaust to try to keep up or floating in the water.
I realize that hull mods were mentioned. Tubbies, a ride plate and intake would help a lot. I am working on those parts but do not have them yet. The stock hull is a fight anyway.
I have only ridden a stock 750 (sxi, pro, w/e, call it a 750) once and it felt very nose heavy to me. This was early in my riding and I can't acurately describe the ride. At the time I didn't like it much because I was looking for free ride and never rode bouys. I have a SJ because i want freeride and bouys. I fell from my short memories of that short experience that the 750 would do a lot more of what you are after. Turn with the nose and carve through the corners. The SJ has a lot of pop which makes it good for free ride but it is hard to get it to dig into the water and hook up through the rough chop.
Most of my expierence has been on 750X2, 650sx and SJ. I rode SXR a few times and 750SX once. I know my answer doesn't fully provide what your looking for, but I am trying to give you something more than "Buy something you can't use in the class and doesn't slightly answer your question."
Long story short, I can't say a lot about the 750sx because I don't know about it very much. I love to ride anything I can ride and would love to get more time on one and tell you all about it but I don't have access. Hopefully the SJ info I gave you will help. I don't think the SJ is the right boat for you. I wish I could ride a 750 and give you a difinitive.
Edit: Btw, I'm 5'11" 180 lbs. I have found getting weight on the nose of the SJ very much changes the ride. I can hit the corners hard by getting my weight even with the bars with the ski tilted and my butt almost in the water. Putting a lot of weight on the bars pushing the nose down into and during the corner gives a lot of hook up, while shifting weight more neutral or back while completing and exiting the corner provides a lot of hookup and control to exit the corner. This is what makes the cornering so fatiguing. I think the right ride plate and a -2 pole would greatly improve my situation.
I can't push on the bars hard enough in the heavy chop to keep the nose down. I think an extended ride plate would help emensely, maybe hull extensions. A scoop grate would help keep my pump loaded. I'm working on tyring to get these parts now. The only thing is that a stock SXR doesn't need these parts to hook up, it just does. I don't know how much the 750 is like the SXR but I only know about the SXR. Maybe the 750 hooks up better than the SJ even if it's not like the SXR. It's the hook up that's killing me. I can't accelerate or corner in the air.