I’ve recently been doing some engine noise reduction efforts. The N42 has space for an extra engine cover which we did not get with the boat.
There’s plenty of science online behind it but my understanding of the general principals are to reduce vibration and noise absorption..
For the vibration it’s likely that there are areas past the engine itself, ie gearbox, shaft that are exposed to the floor panels. You could look to add the material you’ve used or something like silent coat (https://www.amazon.co.uk/Silent-Coat-187x250mm-Deadening-Proofing/dp/B00NVF9OIA) which is designed for cars.
Things that are likely to vibrate like inspection hatches in the floor we have 4mm one sided foam tape on which stops them going crazy. I think you can get thinner but it’s what was on hand. Something like the above will help reduce the amount of sound that can start from vibration.
My basic understanding is that you need something soft, then something hard, then something medium hard. You can layer this approach to enhance.
We added 12mm plywood with a heat/sound absorption barrier on the bottom similar to the one you have described and then on top placed heavy rubber mat similar but a little cheaper than the ‘high mass polymer barrier’ sold by asap. There is then another layer of absorption and whatever ply is on the top.
So I’d say the answer isn’t one thing it’s multiple layers, ideally with something relatively solid like 6-12mm ply in-between.
We did the first round recently and it’s made a decent improvement. I don’t have a DB meter and though we tested with a phone it , I now need to locate the remaining ‘opportunities’ where the engine is basically vibrating straight through into the aft cabin which will require likely more time of me upside down in the engine bay.
I’m not sure that changing the shock mounts will help that much though they do often corrode out. Can’t talk to the exhaust though the elbow on the Yanmars is recommended to be changed every 2 years.
Not sure if that’s helpful…