Coming in at 1001, then.
Matt, I used the pink batts material behind the teak veneer I relined the front cabin where the PO lost momentum after ripping out the old water-stained lining. The thing is it is retained nicely by the teak veneered ply. In that situation it works well, deadening the slapping of water on the hull and preventing condensation, and keeping the front cabin warmer in cold weather, cooler in the heat. How would you achieve the same in the engine room to fix it in place. You want it sealed off, or the tiny fibreglass fibres permeate everywhere.
That (rather expensive) shiny sound deadening material at least has the advantage of being self-contained, and tacky-backed with adhesive, so easy to fix, and the silvered surface reflects heat as well as sound back into the ER,. There is usually 2 layers of foam of a different density separated by a sheet of something between them to further increase the deadening effect as well. To get the same deadening effect, you would need much thicker batts and some way of covering and sealing it off, and at the same time fixing it to the hull, and underfloor surfaces. Could be tricky - possible with imagination, and I know you have that, but tricky…
Bugger it Auscan, you pipped me...