o2 simulator
#2
http://www.autotube.net/ I got one of these to rid myself of the CEL , really didn't notice any improvement in either
#3
the front (pre-cat) o2 sensor is needed by the computer to control the fuel/air mix, by sniffing the emissions. your truck probably won't run very well w/o the front 02 sensor. if it does run, it'll probably run a little rich, which is worse for mpg, but might be ok for power.
the 02 sim replaces the back (post-cat) o2 sensor in a setup that has no catalytic converter. the job of the second 02 is to sniff the exhaust and ensure that the post cat emissions are cleaner than the pre-cat emissions. the simulator provides a computer signal that says everything is hunky-dory. without this post-cat signal, the computer will set an annoying CEL. so why would you remove the cat - free-er flowing exhaust, avoid cost of replacing a bad one, dual exhausts, headers, etc...
sorry, there's really not anything that gives you more power and more mpg at the same time.