> does the nouveau driver these days allow enabling higher power/speed?

My understanding is that, yes, with the Nouveau GSP code in Kernel 6.7.

https://www.phoronix.com/news/Nouveau-GSP-Merged-Linux-6.7

> - how would I switch between the OpenGL and Zink implementations when running an app today?

There are environment variables to control this. Or you just install one driver. Usually Zink is cool when you don't have an OpenGL driver for a specific hardware. You write a Vulkan driver and then Zink is a GL implementation that uses Vulkan.