[sperry]

That RRE quote sounds like a sales pitch, not anything scientific. Granted I'm just trying to remember whit from a physics class I took like 9 year ago, but I wouldn't want the header/up-pipe to expand in size because that would slow flow and drop pressure in the pipe.

There's a reason no one uses an up-pipe on the WRX significantly larger in diameter than the stock pipe.... it slows down the flow too much. I don't know the details of your motor, but I would expect sticking with 2.5" for the entire up-pipe would help spool time. The only reason to have a pipe that expands to 3" is to provide a smooth transition to a large turbo that uses a 3" exhaust input, and that expansion should be as short as possible while remaining smooth.