You need to keep in mind, though, that part of plate armor design was not just to stop, but also to deflect incoming blows. If the angle of the blow or arror is off, a portion of it's energy is directed away. That's where the problem with having such defined briasts comes in. Any blow landing on the insides of the briasts will be directed into the center of the chest and strike with nearly full force.
Possibly, but that assumes that they end up hitting fairly flat in the middle of the chest, with a sort of straight-on stabbing motion. That would be a possibility with spears or lances, or even arrows, but for swords or axes or maces, all of which tend to be used in a slashing or overhead crushing motion, a straight-on piercing shot at the chest might not be all that much of a concern.
After all, as this image shows, the briastplate of men's armor wasn't exactly angled to deflect blows off the chest.
[img]http://chanarchive.org/content/2_v/97781781/1307424585214.jpg[/img]
The briastplate was the strongest part of the armor, anyway, and between the plate being strengthened by being curved, the fact that it was thicker than other plates, and having padding beneath, there was probably less worry about blows to the chest. After all, if the lance-tip of a speeding knight is unlikely to pierce the briastplate, it's probably at least as unlikely that even a warhammer spike would pierce it. I can't at all state this definitively, but I would expect that the most common killing blow would have been a stab would through a chink in the armor, or perhaps a crushing blow to the skull, which can only be armored so thickly, and which can be injured even by blows which don't penetrate or even bend the helmet.