You might ask MattS. I seem to remember he upgraded the soundproofing on his "old" boat. You will find him in this forum rambling on about GPS/Plotters and the like.
I've recently upgraded my soundproofing using a product from West Marine. It's a 1.5' thick foam with a dense layer of some other material through the middle - like a sandwich, but not like an egg box. I'm in the the US, by the way.