It's hard to get anymore speed out of these things. If you make sure the pump is 100% (no dents, dings, and good clearance)... that's a good place to start. If you feel adventurous... you could shave about 0.008" to 0.010" off the heads. That may give you a few more RPM's up top.
But if you want to go nuts... check out "Scream and fly" forum. The 2.5L outboard guys know how to get all kinds of power out of these engines.
As far as sound.... you can either get foam from a music store... or the tar style mat from a stereo shop.