It has been my experience that the results from having a forcing cone lengthened depend in large part on how well it's done. A longer forcing cone allows a shot charge a longer transition time and helps to stabilize it.
In the old days, we only thought of lead shot and longer forcing cones were thought to help that type of shot keep from being deformed (on the front side) upon entering the smaller bore too quickly. Cushioning wads were supposed to keep the shot (on the back side) from being deformed upon set-back.
Shotshell makers offered harder lead shot, high in antimony, to help keep it round and resist the forces exerted upon it on both ends of the shot charge. That along with better wads helped to some extent.
When tungsten-based shot came along, it was supposed that since the shot was so hard, a longer forcing cone was no longer important. Most shotshell makers at that time concentrated on keeping the shot away from the bore with thick wads and recommending very open chokes.
As time went by and everyone learned more about the performance of tungsten-based shot, wads got thinner and chokes got tighter but hardly anyone thought about the effect of stabilizing shot charges and the effects of harmonics on shotgun barrels.
When you have some time and it will take some time, you might want to read this article about the effect of harmonics on shotgun barrels http://allaboutshooting.com/blogs/blog/98758599-harmonics-in-shotgun-barrels
The major effect of lengthening a forcing cone on tungsten-based shot is that it allows a longer transition period, a longer time for the shot charge to stabilize, before entering the bore. If however that transition is still too abrupt or poorly done, it will not benefit the shot charge.
Not every gunsmith has the knowledge or skills necessary to lengthen a forcing cone properly. If not properly done, no benefit may be seen and it may even worsen performance.
Shotgun barrels can be precise instruments for delivering a shot charge or they can be "scatter guns", all depending upon how much care is taken in the "internal geometry" of them.