okay, if you play really optimally and minimize your blocks, you can get higher, although I'm again lagging out at 5120m
I'm willing to bet it's possible to go higher without unbearable lag, just really difficult
Is it possible to go beyond 20480m? Probably. I don't know if I'm willing to find out though... at this point, even in fullscreen, I can barely see the stack at all (which is approximately 80 blocks wide at its thinnest). I'm not sure how I even placed the last block correctly to get past 10240m.
In theory, I used twice as many blocks than would be strictly necessary, so maybe the next level is possible. You would have to make a much thinner tower than I did though, which would mean it'd probably be actually invisible at this point, making placing blocks accurately pretty much impossible