I believe the only way to cut through the hype and find the answers for yourself is not by comparing similar guns that have had the bores treated differently, but by taking one gun, and testing it before and after over boring, f.c. lengthening, etc.
SRH
Bob Brister did just that, and reported on the results--as well as dealing with forcing cones and chokes--in his "Shotgunning" book. See the chapter entitled "Barrels, Chokes, and Forcing Cones". He's pretty clearly a believer in overboring and longer cones.
