Muffler losses can sometimes be "tuned out" for the most part, it has to do a lot with valve and ignition timing..
I have seen some well built motors work very well with some pretty restrictive exhaust systems including mufflers, Shot Guns work pretty well, but don't quiet things down very much and most rangers don't like em...I had a set (had

) that had been modified with progressive springs and had a conical shaped insert in the pipe that were good for about 12 db and cost the engine about 100+/- rpm that was able to be tuned with the aid of a Jesel timing set (external belt) and a dash mounted timing module..
The coolest muffler system I have seen on headers is a cross pipe deal with two Borla mufflers sticking out the back of the boat, you could have a normal conversation while a 500+ inch big block was idling right next to you

GT