I'm doing some tests, but when I went to height 500,000 everything was incredibly forgeted. I'm getting more precise numbers now, but it appears everything goes haywire at extreme heights.
At 350,000 collision is almost unusable. At 400,000 visual effects start happening (that persist until reconnect).
Anyway, these are extremely unrealistic numbers. But, at a minimum of 10,000 collision begins to deteriorate.