Salaam... Bro... The Basic role of Muffler is very simple.. to produce raging sound ... 2nd role ... it increases the car power a lil..... 3rd role.. the consumption also inc a lil... (but just a lil)... There is one disadvantage to loud muffler on Efi car though..... Most of the Efi cars have a knock sensor in them... this sensor detects knocks by the volume of sound it is receiving from the engine.... Racers usually get rid of this sensor as when your are racing the loud Sound of all the other cars cause this sensor to report "Knock" to the ECU and thus ECU will go to the safe mode for ur car... Resulting for increases in Fuel consumption and decreases in car performance.. and most the fuel will go wasted trying to coold the engine (which wasn`t hot) so racers usually get rid of this sensor...
So... it may also happed if you use a lout muffler on your Efi car... the sensor may start reporting "Piston Knock" to the ECU as a result... (you know what will happen)....
Hope I have answered ur Question in detail...