This episode of Engine Masters may not look at tailpipes only, but it gives a good idea of 3" vs 2-1/2". I have 2-1/4" tailpipes on my 67. I'm not so worried about street performance though, since I don't street race. I uncork with 3" cutouts at the track.
https://www.youtube.com/watch?v=_PVXvHkr-Vs 
Thats a great vid.. BUT.. he has the 3" all the way
out of the muffs... not changing from a 2.5 out the
back... he wants to change the tail pipe... and the muff
difference is a major part... so its not apples to apples
