Hollowing out a catalytic converter or replacing it with a test pipe or straight section of pipe is a violation of Federal Law punishable by fines in excess of $25,000 and/or jail time for both the vehicle owner and the person doing the work. Never do this if you intend on re-selling your car! For More Read: USLaw

MAGNAFLOW is now offerring complete exhaust manifold with pre-cats: MagnaflowCatalytic Click on 49 State OR California, input your car info, scroll down to chart.

Removing a catalytic converter will result, at most, in 5 HP in power improvement. However, hollowing out a catalytic converter will cause resonance problems. The symptom is most often seen as a hesitation, popping, or backfire at or about 3000-4000 RPM. Upon hard acceleration, the engine will appear to cut out for a split second as RPMs drop and a split second later recover and continue past 3000-4000 RPM. Additional symptoms are a loud and continual popping sound when the vehicle is allowed to decelerate under load and a whooshing sound that is delayed from or following the engine speed. This is due to the open chamber within the hollowed out catalytic converter case. It loads up and releases pressure in the exhaust system that acts almost as a capacitor. The open chamber allows the formation of standing waves within the chamber which reflects back toward the engine and causes the hesitation and miss in the RPM range.
The symptoms become more pronounced as the exhaust system is made freer flowing, meaning that hollowing out a cat with the stock exhaust may cause mild annoyance, while it becomes a serious problem when a header and performance exhaust system are installed, mimicking a ignition miss or backfire. This is probably the best reason not to hollow out a catalytic converter.
You may also get O2 sensor cel with gutting cats.

If you gut it and don't get all the stuff out, it will re-enter your intake via the EGR valve and ruin your engine by putting little bits in your cylinders and oil.

A good way to add more power is to buy an aftermarket hi-flow Cat 1/4" diameter wider than stock to match your cat back system! On a 2006 Mazda 6 V6 3.0 ltr there is only 1 main cat @ rear ( 2 pre-cats). The main cat is 2.25" inlet and outlet. Most cat back aftermarket exhausts are 2.5", you should also change the main cat to a 2.5" one which is Magnaflow part # 94106. This is a nice one as well: 54906. OEM Grade: ClickHere. Change the last digit to 5 from 6 for 2.25"

For more on 2006, 2007 V6 Exhaust see: https://www.mazdaforum.com/forum/maz...-design-28940/

This is why Headers is also not a good reason. When you install headers, you do not have room to install pre-cats, so most people just have main ones in rear (these main ones must also be metallic due to no pre-cats and people have had problems-- ClickHere.) A Quote: "I forgot to mention that when buying a high-flow cat for your header system it is recommended to get a metallic cat as members have had issues with melting non metallic/ceramic cats. Just thought that info would be rather useful".
Also the pre-cats (on my 3.0 ltr at least) are part of the exhaust manifold as a unit (see pic below).

 

IF you have a bad catalytic converter you may want to try this:

Found this great new Video that says with a Code P0420 will come up with only 5% inefficiency. He says you can clean it by putting 1 gallon of lacquer thinner in 1/2 tank of gas and drive it at higher rpms for 150 miles. Plan B is to remove the Catalytic and put it in a bucket with laundry detergent and water and let it soak overnight.

You may want to try this before buying a new Catalytic converter!