If any improvement is made, it's going to be maybe 1 or 1.5 mpg. It's not worth the effort to convert, nor is it worth the damage to the environment.

You'll get better mileage by changing your driving habits. Don't jump on the throttle all the time, and upshift earlier.

Lastly, removing your catalytic converter is illegal. Period.
The cat is there for a reason - to keep our air breathable. Please don't remove it.