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The noise won't really influence FE at all. I say a statistic that the energy in teh soundwaves or someone yelling would take several years to produce the same energy as is in a cup of hot coffee.
dampening sound creation is probably the last place anyone will ever go to save energy(unless its only output is sound, speakers).
The trick to exhausts is you want 2 things.
1.) you want the exhaust to get away from the manifold without causing any backpressure.
2.) you don't want the exhaust to slow down in the pipes because of friction and create backpressure.
Issue one is avoided by making sure the pipes are not too large right out of the manifold. If they are the gas cools too quickly and decelerates creating(pretty marginal) backpressure.
Issue two is avoided by making the pipe as generally short as possible. On cars its got to go through cats and mufflers and then out from under the car somewhere, so its not very short.
Its pretty irrelevant unless you are running with the cat ripped out. If that is the case you want something thats just an open pipe.
Quieter doesn't necessarily mean more FE. Your muffler is a big box with baffles in it that dampens the sounds. . .but it also decelerates the air substantially. This is the same reason why silencers slow down bullets.
For a car your exhaust gases are accelerating for about 16-20 inches from the manifold. Any distance after that and they are decelerating and creating backpressure(pretty negligible). So unless you don't have a catalytic converter anymore don't worry about it as long as the exhaust gets out of the muffler.
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