I have never seen 20 MPG or anything over 16 for that matter, but i'm driving around in a tank. It's just the price i pay for driving what i like.
If you were to try disabling half of the cylinders on your PSD, you would have bells and buzzers going off everywhere, unless you spent some time re-engineering the CPU. The cost of doing what you are talking about would far outweigh the simple mods it would take to get you in the higher teens. Just look around here, and you will find several options that could help.