To my sense, they are pretty light up front and can handle the extra couple of ounces. This certainly doesn't apply to all gas guns, but I think it does to the 391. I generally select barrel length more for balance than sight picture. I suggest you do the same. Pick a barrel length that seems to balance nicely for you and then live with whatever length that works out to be.
Balance is everything in shotgunning.
When I hefted the 391 synthetic at the SHOT show, I felt that it was a heavier gun than the 391 wood stocked field gun. I couldn't say exactly how much, but it felt a bit heavier and seemed to move the balance point back a bit. Factor that into your barrel length requirements also. I think too that barrel length selection also depends somewhat on your individual height and strength. To some extent the overall length of the gun ought to fit the shooter.