Its not possible to generalize this way. The proper level of effort depends on the task, the size of the code, how much context is easily available versus deeply hidden, how novel the ask is versus formulaic, etc, etc.

Medium usually gets the job done quickly and correctly for me. I use high when a request is out of the ordinary and has some tricks to consider. I haven't seen better results going higher.

That's for coding at least. For creating images (SVG, 3D models, etc), medium does usually suck, and every level above improves it.

Depends on what you do. Medium is really good for implementing yet another feature in my code, really.