I think there should be a middle ground there. You can know a lot about code optimization but if you don't have an OK understanding of the lower parts, you will

I think there should be a middle ground there. You can know a lot about code optimization but if you don't have an OK understanding of the lower parts, you will spend lots of hours discussing with sysadmins or netadmins about how it is their or your fault that you application is not behaving harmoniously with the OS, for example. Being a jack of all trades is not a bad thing most of the times. It helps give you a better understanding in the end.