I have seen 860W AllStart CMH retrofits for 1000W QMH lamps, but that is the biggest I have seen. Well I did see a 1000W CMH lamp, but it had 2 arc tubes in series so it doesn't count lol.

Anyway, is there an actual 1000W CMH lamp? Or maybe something even higher? I did some searches here but didn't come up with anything significant. And if not, why? Does CMH only offer significantly greater efficiencies at smaller wattages?

 Not that I've ever seen .

The higher the wattage, the lower the fill operating pressure and temperature. Plus the ceramic technology becomes exponentially more difficult to produce when the sizes go up.
So it is mainly the lower power range, where the temperatures are the highest, what benefits the most and suffers from the manufacturing difficulties (so the cost becoming high) the least.