Just for some food for thought, I'll add a story:
Back in '05 when I first got my 760T, it was totally stock apart from a boost gauge, still auto. The exhaust was a stock 2.25" dump pipe, into an NA 2" system, and for some reason a 1.75" tailpipe. The turbo used to glow orange easily when it had this exhaust.
I put it on a dyno to get a measurement of what I was starting with, a whopping 77kw, with 7psi boost.
I then unbolted the exhaust at the flange at the end of the stock dump pipe, and did another run, this time it made 100kw!
I then got a 3" turbo back system made up with no cat and two mufflers. The boost went up to 8psi without me changing anything, and it dyno'd at 96kw.
With a few other mods and 12psi it eventually made 163kw, so I consider a full 3" system a worthwhile supporting mod.