I did the engine hatch, the trans hatch and the panel over the gas tank. The panel over the gas tank for sure made the least amount of impact for me and I would not recommend doing it. We used a layer of dynamat and a layer of the foamy on top of the dynamat in all places.

The biggest difference out of all of these is the free one...just take your boat cover and stuff it under the rear seat that's right above the v-drive cover. May not look classy but you can stuff it in there far enough that it's not stepped on and it made the biggest difference (by far) for our sound proofing problems.