You need to do a little scripting, or some Visual Logic. Basic idea is:
- set your text centered on the x and center justified (as you did)
- set Y Pivot to be at max
- position text object at 960, 540, for a 1080 comp.
- use Visual Logic to move the text box up half of the height of the bounding box
The "Value" block number is 540, for half the height of a 1080 tall composition. The values shown in my image are with 3 lines of text, and they auto-center on the Y axis, as soon as you type a line break.
Check out the VL blocks in the image.

#XPression