If I remove my cats will my MPG increase?

I asked my tuner and he said it would decrease which is the opposite of what I heard for pretty much my whole life of modding cars

 

removing them makes the truck louder. As for mpg going up eh i can't say yes or no due to never keeping my foot out of the throttle.

It can help i'm sure but it's all on how you drive it i guess

 

I'm almost certain that it will help but that guy confused me.
I think it's time for a new tuner

 

It makes zero difference.

I gained nothing when I added headers to my 3800 power car and gained nothing when I added a offroad y-pipe to my 06 sierra. With both have the rear o2 sensor's disabled in the tune, still only the front o2 sensors are doing the calculations and nothing else is changing.

The cats are not a restriction at all when you are just cruising around.

 

But there is a performance difference right