every little bit counts when your trying to save gas money when you'll own something that has a hungry 5.4L V8 connected to a extra cab truck for a daily driver for a few years lol.
Removing the catalytic converters for any year is highly illegal. OBDII just makes it harder to get away with it since everything is computerized.
The best way to gain mileage and performance is to lose dead weight. Interior trim, seats, headliners etc. I would say the spare, but it would be a dumb idea if you're daily driving the car.