I've seen the question asked a few times, "Will my fuel economy improve/worsen with mods x, y, and z?"
For the Berk HFCs and F.I. CBE, at least, I have an answer of sorts. The best combined fuel economy I ever saw when I was completely stock was around 18.5 mpg. This was not driving conservatively or aggressively all the time, but more a combination of the two depending on circumstances.
Now that I've had the HFCs and exhaust installed for a while (and reset the ECU after installing them), I decided to reset the mpg calculator on my Z and see what my numbers would be without the pre-mod information tarnishing the results. Surprisingly, my average after a few hundred miles has been 21.5mpg - a full 3 mpg improvement! Furthermore, I definitely go WOT more often and generally drive harder because I like hearing myself now that I've got this beastly sounding setup.
Anyway, just figured this information might be useful for those who were curious.