[drcampbell]

The cat represents a substantial restriction to flow. Removing that restriction allows for better breathing and can improve efficiency ...

This is relevant only when the engine is operating at a high power setting -- at or near redline rpm and at or near pedal-to-the-metal.

When cruising at a moderate power setting, which is where 99% of your fuel economy is determined, the exhaust temperature & velocity aren't terribly high. The muffler and catalytic converter (provided it's in good condition, not damaged or clogged) impose a negligible amount of back pressure. Removing them would only be "solving" a problem that you do not have.

Removing them will increase the amount of power the engine develops, but power and fuel economy are two completely different things.

im from fl so emissions really isnt a problem ...

Emission law is federal. It's illegal to remove a catalytic converter everywhere in the United States. Perhaps it isn't enforced in Florida today, but it's likely that it will be enforced tomorrow, and you may wish to drive through other states that aren't so lax. Plan ahead, don't plan ahead -- your choice.

Hollowing out a catalytic converter, reinstalling the empty shell and avoiding detection is a thing of the past. When a catalytic converter is functional, the temperature of the exhaust increases as it passes through. This temperature rise (or lack of temperature rise) can now be observed with a $99 infrared thermometer as part of a "visual" inspection. (Courts have ruled that observations in the IR spectrum fall within the definition of "visual" and "in plain sight") So any DUI checkpoint, roadside stop, weigh-station visit, impromptu safety check or plate-renewal visual inspection could turn into a fine for "Tampering: to modify, remove, render inoperative, cause to be removed, or make less operative any air pollution emission control device or element of design installed on a motor vehicle or motor vehicle engine which results in an increase in emissions beyond established federal motor vehicle standards." And fines for deliberately tampering and attempting to conceal it can be steep.

 

[drcampbell]

... There is a temp rise at the cat ... because it is a restriction ...

First of all, that's wrong. As Theiba says, the temperature rise is due to the oxidation (burning) of CO and HC, and the chemical reduction of NOₓ, yielding CO₂, H₂O, N₂, O₂ and heat.*

Second, you've got it backwards:
When a gas is expanded and its pressure is reduced, its temperature is reduced.
When a gas is compressed and its pressure is increased, its temperature is increased.

* CO - Carbon Monoxide
HC - (unburned) HydroCarbons
NOₓ - several different Oxides of Nitrogen
CO₂ - Carbon Dioxide
N₂ - Nitrogen
O₂ - Oxygen