We have a 32 foot boat with an Atomic 4 which was bolted right on the engine bed rails, without engine mounts. It is under the stairs, open to the space under the cockpit.
I added sound insulation from here...
LED lighting, soundproof, Sailor's Solutions Inc.
around the three sides of the engine, but it is still open under the cockpit. I used a decibel meter to check the difference. I do not remember the actual numbers, but the drop on the meter was not as much as I had hoped. I was pleasantly surprised however at how the actual pitch and tone changed, and made the noise much less intrusive and much more bearable on long motoring days. A lot of the noise was from the vibrations though, not actual engine noise as evidenced by the fact the V birth was likely the loudest part of the boat!
Since then I have replaced the engine with another Atomic 4 I rebuilt, and added real engine mounts with rubber isolators. This drastically reduced the over all noise as it cut down on the vibrations immensely. Now that the vibration noise is reduced, I'm curious to know how much of a difference the foam is making, but I'm not about to pull it out to check!
Someday I want to add a rear wall to the engine compartment to cut down on noise reverberating through the space under the cockpit and thundering in the quarter births, but that's low on the list now.
So I guess overall I would say if the cost is not much to you, go ahead and give it a try, it will likely at least change the tone and make a difference that way, and might reduce the overall noise as well, especially if your engine has rubber mounts on it.