Running a 4" system you can see an adverage of about 250 degrees compared to a stock system. Because most diesel truck owners are running some sort of programer on your vehicle the EGT numbers might be less than of a stock vehicle.
Its a waste of money IMO. Take out/gut the cat and your just at good as a 4" trust me I did it on my 05 LLY. I then sold the 4" for the same price and saved me some $ that can be used elsewhere (diamond eye down pipe) and 2 two tanks of diesel.
With the 4" i gained 1 mpg hand calculated egt drop 250*
With the 3.5 stock with cat removed, gained 1 mpg hand calculated egt drop 200*
With the 3.5 stock with cat removed, diamond eye down pipe, gained 1.7 mpg hand calculated egt 240*
i just put a 5" straight pipe on 2 days ago from my fully stock exhaust. since you have an lb7 i can def tell you its a world of difference, and way loader . but i dont know what the stock system is like without the cat and muffler either