Well, to each his own. You can cut your springs for free to test it out. If you dont like it get some from the bone yard.
The past few vehicles ive lowered it was up to 5 mpg. I lowered my sidekick and also went up 3 tire sizes, luckly I am at the same mpg in the end.
I use either eibach or tanabe springs. The nfs are good for lowering and firmness, df will let the car bottom out and ride the bump stops.
As for best bang for your buck for instant mpg increase, just block off your radiator.