Same thing happened to my 2004. I added the Volant and duals and I lost a couple. But I think mine was directly related to the increased pressure of my right foot in response to loss of backpressure on the 5.3L. So I did the sensible thing.
I bought a supercharger!!!!!!!&#3 3;
The supercharger and tune added around 3 mpg with better fuel tables.... For about a day. I just couldn't get rid of that twitch in my right foot!

Seriously, the tune can do nothing but help....