If 600% is the maximum, then what does this mean?

> Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.

I think it tracks how well the materials they're using move heat between stages or lose it to the atmosphere. Their modelling (supplement figure 2, mentioned on page 4) depends on their specific construction. I wonder what it would do with gold as the plate material and aerogel everywhere else...