It always helps to polish the inside of a barrel. Legenthing a forcing cone is a toss up. It might help and it might not when using hard shot. I had my cone legenthened on a Benelli M2 and it did help. It always helps if you only shoot lead shot.
If you're shooting a Browning with the Inv-plus all you need to do is polish the inside of the barrel and install a Indian Creek .670.