Well, of course if you have your feet planted in the same spot a 32" bbl will get you a half foot closer to your target than a 26" bbl will. Velocity does not actually level out in a shotgun at 4", but each added inch of bbl has less & less gain as you get much beyond that. Only guns with very light powder charges such as the .22 rim fires will reach a point where the velocity gain will cease & actually begin dropping as bbl length increases. With the .22LR max occurs at about 19" of length. It will stay fairly constant for a few more inches & then begin to fall off.
In a shotgun there is of course much less difference in the velocity of a 32" versus a 30" than between a 22" versus a 20", but a difference does still exist & will for virtually any bbl length one is likely to contemplate.
It would take a large amount of well carried out pattern testing to prove a reliable statistic of pattern density between bbls of differing length but choked the same. I will note though that while not an absolute given, much of the "Lore" surrounding long versus short bbl stemmed around the fact that short bbls were traditionally more open choked than were long ones, their basic intent of use being different.
