It might not be "good" but is it not better than a leather tunic?
Several things to consider, according to the swordswoman/stage fighter/stunt person who made the video I posted:
1. Does it affect your ability to swing your weapon correctly? Highly decorative DD cups, yeah, that totally changes what you can do to attack, parry and basically fight with your weapon.
2. Does it affect your ability to move in other ways? is the corset part flexible, and allow your sternum, breathing areas to move. If you can't run because you can't breath, that's a problem.
3. Does it add extra weight. A lot of plate armor was designed to shift some of the weight to the hips, so it wasn't all hanging off the shoulders. That will tire you out a lot faster in a fight, than having the weight displaced across various parts of the body.
4. Leather tunic isn't the the only other option. Chain or maille is flexible and doesn't add much if any weight over plate. Much preferable to a leather tunic. Boiled leather is fairly hard. I'd take it over being tired out, or not able to swing a weapon, in a fight.
5. If we're into more of a musketeer era, Porthos, D'artangnen etc... I'll take the leather tunic. Plate is useless against a musket ball. Close quarters fighting I want agility more than weight and movement restrictions.
Yes, I'm a bit of a swords and armour nerd.