Honestly, you're probably better off using Billy Ray's method.
My method was this - remove vest, cut clear plastic off. That's it.
This allows for the chest armor to SLIGHTLY 'float', meaning the figure can contort his abs/chest a bit more. In addition, as I was screwing the backplate back on, I pulled the lower portion of the vest together as snug as possible, since the screws will push the vest together a bit more that way. Nothing scientific about it.
You will find that there's not much we can do to increase poseability in the chest area, as no matter which modification method you chose, you will continue to be hindered by the material of the armor pieces themselves. That's why I'm reluctant to pull the pieces off and just glue them back on. They are not pliable, in fact they're quite rigid.