@gcavey The Slider Wide widget area is already centered. It’s the Slider Plugin you are using that doesn’t center or align in the middle itself which is where a lot of people get confused. I haven’t seen a good way to make the plugin center, and that is due to the way its coded.
The reason the slider “appears” centered inside of the regular “Slider” area is because the regular slider widget is 1140px wide and is always centered. The full width slider wide widget area is also centered, but it has no width. When you add something that doesn’t center inside of a full width area it will either left align or right align. The only thing you could try is adding a large width to the slider wide (or the regular slider widget) so the slider plugin will appear centered when viewed in a browser. The only other option is to try a different plugin that actually centers on its own.