^Easy big fella. I base my opinion on 28 years of drag racing and 7 years of open track racing and moding various makes and models of cars, forced induction, naturally aspirated, gas, and diesel.
Your argument as to why would they recommend a 3" over a 2.5", when they make both, makes me wonder why even make the 2.5" if the 3" is better all around. It's because it is not. If you're going to pull out all the stops for peak HP, a 3" is the way to go. If you are going to do the basics, tune, CAI, intercooler. Then the 2.5" will give you better throttle response and torque. I am not saying that a 3" won't make an improvement over stock. It is more than necessary for most mildy modded cars.
As of a few years ago the diesel truck land speed record was held with a truck that had a single 4" turbo back exhaust. That truck made well over 1,000hp. I have a friend who would run low 10's with a turbo Mustang, using a dual 2.5" exhaust. Bigger is not always better or necessary. I am sure a 2.5" will have plenty of flow for my Fiesta. If you choose to run the 3", that's cool.