From a Cubase/Nuendo standpoint, I am not aware of a change in performance between 10.8/10.9. That said, typically an update in an operating system requires more power to run it.
We are not supporting 10.10 yet in any of our products. Historically we have not officially supported beta versions of operating systems. We generally wait until the retail version is released then we have often released updates for the current versions of the software to become compatible with the new Operating System. Sometimes this has required a new version of the software, depending on what the changes in the new OS were. I hope this information is helpful.