There's generally a pretty quick falloff of how much help knowledge of each layer under you generally provides as you go deeper.

That being said, if you're writing in C, having a pretty good idea of how a cpu generally executes instructions is pretty key to success I'd say.

Agreed, also depends on the scale you are working at.

If you are a tiny startup, the marginal gains from these optimizations matter a lot less than if you are Netflix.