Keep the cat lose the muffler.

 

My stock catalytic converter is back in my truck.
The drone while towing our 10k lbs 5th wheel was unbearable, plus the lack of cat greatly increased the diesel fumes that my wife didn't like.

I put 560 RWHP/1150RWTQ thru my cat!

 

When I bought my 05 the previous owner had removed everything from the turbo back. He installed a 5 inch system and a fake muffler. It was loud but I have always been told this opens up the exhaust and I would get better power and MPG. After the first tow experience with my 5th wheel I removed all of this crap because the drone and noise was beyond belief!! It actually hurt your teeth if sitting in the back seat while towing. I called the dealership and threatened lawsuit over my truck being configured for a CAT but it was removed and this was not made clear to me on buying the truck. The truck was represented as being unmodified except for a loud muffler. Of course it went beyond just being the muffler. He agreed to send me the OEM Cat and I would install it at my expense. I bought a stock muffler, and stock 4 inch exhaust.

After installing all this stuff I started checking my mileage because I had hand calculated several tanks and knew the truck delivered 21 highway non towing at Interstate speeds of 65. Then we towed the trailer again on a short trip and to my amazement there was little or no recordable difference in mpg and power. I'm sure EGT temps would be higher but seat of the pants experience was not apparent. All this expensive crap just made the truck annoying. What a waste of money. These trucks are engineered to be strong and long lasting. Anything you do is to massage your own ego. Without a dyno test I don't think you will notice any earth shattering performance enchancements. Save your money and buy new tires or do a transmission fluid change. Don't waste $$$$