canpower- i've heard of EFIE's that either amplify the existing signal coming out of the 02 sensor, and i've heard of EFIE's that simply replace the 02 sensor altogether- rendering the one on the vehicle useless.
manipulating the signal from the existing 02 sensor can be useful, and safer, in my opinion, but it brings with it tuning issues.
using an EFIE that altogether replaces the 02 sensor altogether is in my opinion, the way to go. the EFIE has to oscillate a signal from around 0.2 DCV to 0.8 DCV rather quickly, a few times a second i believe.
my car has been seeing gains with just a booster setup, see the link in my profile ().
a third option that works for some and is ENTIRELY cheaper and simpler is the spark-plug non-fouler option. it essentially takes the 02 sensor out of the direct exhaust stream. now for some, it throws a check engine light. if your car determines that the 02 sensor is acting faulty or whatever, it may run in "open loop", a fail-safe mode that enables a person to drive a vehicle while an important sensor is failing. typically, a lot of extra fuel is used, however its a fail safe mode, not a "drive me like this forever" mode.
i'd say get some reliable numbers on fuel milage and consumption. start writing down your figures and establish a baseline. in the meantime, start building a cell to your liking. i built mine entirely from $1.38 SS wall switch plates (Smacks booster style plates) and it works excellent.