I didn’t read the thread.
But you could potentially gain power. There’s not enough info in the thread to make a blanket statement like that.
Things like exhaust design, bends, muffler choice, coatings, connections, etc, all will have an effect. Not to mention that who is to say 2.5" was the optimal diameter to begin with?
Turdblows love free flowing exhausts, put on an open dump if you’re that concerned.
Cliffs : Don’t worry about it.