How much power will I get with 2.5inch piping?

I currently have weld-in flowmaster 40's mufflers (2), and I contacted my muffler shop saying that I can change to a custom 2.5in piping and just weld my existing flowmaster 40's on there. I am doing this for better sound, the man I talked to said "The bigger the pipe, the bigger the boom." So will I get a noticeable sound improvement with 2.5in piping as opposed to my stock 2.25in piping?

Also, do you know how much horsepower (and torque) I'll get with 2.5in pipes with 2 flowmaster 40's?

Thanks