I have run them with no noticable difference in power.  I run 10 X 5 APC props, deflashed and balanced and 7 1/2 % nitro and 29 % oil.
  (24%castor, 5 % Klotz).  I would imagine that if you run them at a dead lean seyying, you might see a more pronounced difference between no muffler and a stock one.  But I run my engines in a hard four cycle setting.  I generally use the muffler(s) to obtain the C G location rather than lead. is  Some tongue mufflers are around0.5 oz and the stock one is a full ounce heavier and there are others in between.  I also use spinners likewise.

 Bigiron

Here is the muffler I used to run on Fox 35's back when people would ask me if it was an St 46.  World's smallest and least effective muffler.  The case it is shown on is not drilled and tapped for the mounting bolts. 

So far as noise abatement, I doubt it did anything.  My idea was to get muffler pressure for the uniflow.  The cut out portion at the rear of the plate is bent into the exhaust to make a little pocket for the pressure tap to draw from.  It did provide noticeable pressure and my stuff ran fine.  I don't think there was any loss of power.