I have a tank in Y: 100km
In editor you can force values into xml bigger (and smaller) than the limit you can see, in this case 9,999km for the xyz values.
The problem here is that a part is able to be placed in Y:100km but the center of mass takes a max value of 9.999km, and if you place the part higher the center of mas doesn't change
The game would probably have issues with any craft that has parts that are more than a few hundred meters apart from each other.