That's kind of the direction that I was thinking also. I do have all the rubber hoses needed with the silent choice installed. I have gone back over them and made sure everything was tight. When I replaced the port manifolds, I had to take all of that off, and I did find one of the exhaust tip through hulls was loose. I siliconed the crap out if it and put it back on. I checked the others, and they were still tight. I do get some water entering the bilge, I just can't seem to find where. All the gimbal/shift bellows are less than a year old, so I doubt it's there, I think it might be a through hull for the mechanical tab or drive indicators, I just can't see it laying on top of the motors.
I think you're on to something with the amount of gas secaping under the platform. The downturns helped the vibration considerably, about 80% better, but you are right, those gasses have to go somewhere. Before I put them on, it would blow so much exhaust out from under the swim platform that it would completely soak the back of my sunpad. Now, not at all. You're right, it has to go somewhere.
I'm leaning very much towards it being the downturns. I think I'm going to have to find another option. I saw a formula a few weeks ago that had stainless extensions on the tails that got the exhaust just past the swim platform. I'm not sure where to go to start researching where to get them. I'm thinking an exhaust shop might, but I seriously doubt they have stainless 4" just laying around. I'm going to see if I can maybe find somewhere that has some stout rubber 4" straight pipe that I can get cut to fit and test before I start throwing money at stainless.
I appreciate you brainstorming with me, I'll keep you posted on how it works. I'll drop the downturns off and test this weekend to see if it goes away.

Is the silent choice still installed and functional? Did you try running the exhaust out the outdrive?

I would try that, also if you have silent choice why bother with the down turns?

The silent choice is installed, but inop. The motors for the diverters have quit working, and I haven't decided if I'm going to repair, or just remove completely to get some weight out of the back of the boat. The downturns were just a stopgap measure to get through the season before I had to decide what to do. The boat sits low enough that I will ahve to remove the y pipes and install a block off plate, and I don't have any way to pull the motors myself, so that's an $800 bill per side just to R and R the motors.

Are your eng compartment vents facing the right way (all fwd on one side and all aft on other side)? I could see if you had a exh leak, and the vents were all facing forward, when you got on plane the eng compartment would pressurize and force air from bilge up into gunwales and cabin water drain tube.

I don't have directional vents on the side of the engine comt. It has one big intake on each side, and that's it.

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the silent choice was just too quiet for me at idle,,as I posted on another thread I installed these tips which are actually diesel truck exhaust tips and they extend 13" out making it easier for the exhaust NOT to get trapped under the swim platform(as you can see in the picture I attached) and I think they look good and now I dont have to hit the silent choice switch every time I come to a no wake and I now have a nice sound out of the exhaust at idle

That is a GREAT idea. Thanks for posting that!

if you look closely you can see the exhaust coming out from under the swim platform at idle..

where did you get/order those from? thay might just be the ticket.