if you check back in one of last years "Shotgun Journal" they addressed the forcing cone issue. using pressure testing, they found that lengthing the cone to make the transition from chamber to barrel longer had little or no effect on pressure.
i did see some speculation in other articles that said that longer forcing cones caused less shot deformation and less balling (where clumps of shot stick together while in flight)
this coming weekend i plan on patterning my .410 handi barrel, i just used it on a quail hunt and had all clean kills, i want to see if a good pattern is indicative of clean kills. whatever the pattern testing shows, i won't touch the barrel, it shoots good !