Good point Nos
Also worth mentioning that the performance of many acoustic foams drops of once the pores start getting filled. Given the amount of grease present inside most outboards, I'd suggest this is going to get smeared all over the foam and therefore pick up all sorts of muck and grime.
Absorbing foams work reasonable well in the engine compartments of larger boats as there is plenty of space to keep grease away from the foam, but as Nos says, space is not a luxury you have under the cowl of an outboard.
If you do decide to try it .. let us know how you get on. Oh, and welcome to the forum by the way