Should I get a performance chip for my 1993 Mercury Grand Marquis?

I drive a low-mileage (31000 miles) 1993 Mercury Grand Marquis. I love my car it is very comfortable. The problem is it is not the greatest on gas, being so large. I have read about different performance chips that supposedly boost fuel efficiency and give your car added horsepower.

I am a little skeptical as to how a little computer thingy can do that. Are there performance chips out there that really work? How do they work? Does anyone have any recommendations of a specific brand of performance chip? How much would I be looking to spend on a performance chip?

My car runs great already, she is old but she is in good condition. I am just looking for a way to improve my fuel efficiency. A boost in horsepower is always good to though :-)

Thanks