It depends on the CAT. If it's in a larger chamber and you remove it you will get a loss of velocity, and reduced flow, due to the large volume.
If you are looking for performance, a high quality full system is a better way to go. As with most mods, your HP gains will come at a cost. Usually that means you will make gains at the top and give up some at the bottom. Header diameter of a performance pipe is usually larger to get increased flow at high RPMs, which usually hampers low and mid-range power. That's why your MPG will drop.
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