I don't know, it sounds like a bad idea. Your making your o2 sensor read less oxegen, how do you know what your actual a/f is? and how do you control it? You can't, and the chance of detonating is high.
I would stay away from it unless you can find a way to control your injection system, or at least watch your a/f ratio.
But to answer your question, yes running lean will increase mpg, if you don't lock your motor up in the process.
Right now Im using a single defouler instead of 2 & things seem a bit different but to early to tell. But the car is driving better according to me & the scan gauge. Ill do a slight fillip soon to see what I got.