Imagine a line following the drive shaft and the ski spinning around that line. Front sponsons stop the roll, improving straight line stability by reducing the roll. They let the water push the side of the ski up so you don't fall into the corner and you can carve harder. I have destroyers $400.
Imagine a line going from top to bottom through the middle of the ski. The ski spins around it like the arm on the compass. Rear sponsons take that sway out of the back end. They improve straight line stability by removing the sway. They also keep the back from sliding out in the corners. I made my own from fiberglass, but you could also cut it out of plexiglass, aluminum, cutting board (like in your kitchen), etc. $15-120. You can also try some of the different ride plates that have channels in them, I have not.
Imagine the ski sitting on a parking hump that goes side to side under the dash. Longer ride plate or hull extensions will take out the porpose. I have a works dominator. Used $100. Works okay straight but no real improvement in the corners. I did try a jet dynamics which seemed better but I didn't get to ride it much.
You are cavitating badly because the stock aluminum prop is a bulky piece of crap. Intake will not help, the stock prop blades are 1/4" thick. It sucks. Replace it.
Don't let my explanations of roll, sway and porpose offend you if you already know what they are. If rather not assume.
In closing, just buy your wife an sxr. I bought one for my wife and she loves it. She can ride my Superjet just fine and did for years before we just got her sxr. The Superjet is great for getting air but she is not going to do any of that. Her sxr is like a Cadillac compared to my Superjet. Unless you wife is seriously racing competitively and wants that hole shot or plans to go aerial, don't try to turn her Superjet into a sxr. Just get the sxr.