Diesel Power Magazine did a build up of a stock 6.7L. They went from the particulate filter back with straight 5" exhaust and said that it offered a little better milage and lowered EGT's but it wasnt much louder than stock.
I dont recall if there was any HP increase but im sure its serveral hundred $ for the system and if you dont have a programmer or other performance parts that would require
the motor to have larger exhaust then you need to decide if its worth the money...
I have a Banks 4" system and i like the sound plus my stock exhaust fell off so i figured might as well go bigger.