Usually, the game won't let you teleport past Y:256 , but I've randomly tried to teleport to a really far position...And the game made it.
Example:

/tp @p ~ 67567485687667446564353457678909789667535645323567567342533454567567 ~

Second thing: You notice that the position you have chosen to teleport is not the same where the player spawns if you see on the F3 menu.
The used RAM floats from a certain number to another , and it makes a cycle.This cycle slows down when you get higher
From random (extremely high) distances it isn't possible to move in the Y axis. But you can still move on X/Z or just teleport to move on Y.
It is noticeable that sometimes Sky is only colored with one color, you can't see the color changing.
In other positions, Sky creates a three-colored nice effect : Example,if you are at night, there are : orange (afternoon-evening) on the bottom , light blue (nearly day) on the middle and Dark blue (night ) on the top.
It is nice to see these "High- Far Empty Lands" , but I still think why the game lets me do this impossible tp.
Third thing: In some positions world border won't work, and you can fall from those positions.In other positions you can't fall, but it is not worldborder 's fault, it is the great position that makes it buggy, since from 2 billion 147 million etc... the world border stops working.

If you want to fix it, then fix.But I still think this high buggy world is nice and makes a mistery-sensation.It isn't laggy
Also, trying to reach 99.999.999. ecc... (9 wrote 71 times) will teleport you to 10^75 ,but the game will say 10E81, which means game will count 5 zeros more than the shown coordinate.

UPDATE
Since of 14w27a , TWO SUNS APPEAR when you go to Y:10^75
It now kicks you out after some minutes.