TW marine do something they call the Trad Engine Silencer - basically it's an expansion box rather than a silencer. I used one as my engine had a straight through exhaust and it certainly took all the harshness out. It is designed so as not to increase the back pressure. It was about £180 from memory, so much more expensive than some of the other suggestions. TW are a very helpful bunch who do know their subject, it would be worth giving them a call.