The 3.0L Duramax Diesel appears to prefer the stock airbox over the aftermarket S&B unit
After driving with the complete S&B system installed, I noticed my fuel mileage seemed to drop to around 20mpg when I was used to seeing 22-23mpg. This prompted the idea to try out the factory airbox with the other aftermarket goodies still intact.
“It's often questioned whether or not aftermarket parts really help. In most cases they have improvements and you just need to decide if those improvements are right for you.”
I did notice an improvement with the S&B airbox installed, once removing the airbox but retaining all of the other pieces, it appears to have not changed my throttle response. This is much appreciated because I prefer a great responding engine in addition to an increase in MPG.