Ok,,, I have a moment to answer this with some time...
I believe any engine issue that did exist is corrected as it was an electronic issue, not mechanical. Thus, if the dealer does a filmware/software update it should resolve the engine concern.
The only in reverse issue was also an electrical issue that has been corrected through the computer. So again, now that the ski has been out a couple of years Seadoo has had the opportunity to resolve these "glitches" for lack of better words.
Now,,, the iS concern is a real one. The new skis it is resolved. I THINK that the 2011 was a model year it was an issue but I need to do some digging to confirm that. I would ask the dealer about it and ask if there is or has been a modification done to eliminate the sinking concern.
Further,, the iS ski does help in slight chop to pretty smooth water. In rough seas or riding it can actually make things worse, especially if the suspension is set up to loose. If the sea is allowed to bottom out on its travel when the ski and seat are coming together, it will be a pretty hard jar as you have two opposing forces coming at each other. The seat would need to be LOCKED or set to a very high resistance to eliminate that concern.
The GTX is a fantastic ski and SUPER comfortable. If I could put that seat on my RXT-X I would do it even if it cost me 500 dollars. The GTX is like riding in a Caddy where the RXT is like riding in a Porsche. The 260 engine is proven and will last for hundreds of hours. It can easily goo 500 hours with proper maintenance.
If at all possible, I suggest riding one ski with the iS and one without it. You will find the iS ski is a bit top heavy by comparison. The smaller and lighter you are this diminishes quickly. But put 200 pounds on the seat and you will feel the difference.
I don't think you will go wrong with the ski. The technology is both a wonderful thing as well as a potential cost detriment. It is not an easy fix when you have issues. But when it is all working as it should, it is an absolute joy....
Any other specific questions? Just ask...