Are you staying in the throttle, or trying to let it idle while under water? You should never let it idle under water. If you are staying in the throttle and it is dying, I think you have a different issue. I go 6' deep with no issues and stock exhaust. I know you said that you have dielectric grease on everything, but are you sure that it is not your spark plug getting wet? Maybe you haven't put dielectric grease in the spark plug boot recently, or even a cracked plug wire? People sometimes blame the exhaust because that is what they can see under water. Your spark plug is at the about the same height as your exhaust. It will sputter and die while under water, and then will stop once you get out. If you are sure that it is the backpressure on the exhaust, then yes, a more free flowing exhaust will help overcome the backpressure from the water. Have you cleaned out your stock exhaust? It may be clogged and that could cause extra backpressure. I have never had that issue with my sportsman, but I did have that on my 700 grizzly. It was so clogged it would barely idle just sitting in my garage.