> I expect that many desert adapted species approach perfection (but have not bothered to verify).

No they have different strategies to minimize water loss that comes with exhanging CO2 & O2 to the atmosphere. For example:

https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism

Portulacaria Afra (elephant bush) is a nice example. It can switch between C3 and CAM photosynthesis pathways as needed.