if you compiled some code and then uncompiled it you would get the most efficient version of it ... ?

Sorta, an optimizing compiler will always trim dead code which isn't needed, but it will also do things that are more efficient but make the code harder to understand like unrolling loops. e.g. you might have some code that says "for numbers 1-100 call some function" the compiler can look at this and say "let's just go ahead and insert 100 calls to that function with the specific number" so instead of a small loop you'll see a big block of function calls almost the same.

Other optimizations will similarly obfuscate the original programmers intent, and thinks like assertions are meant to be optimized out in production code so those won't appear in the de-compiled version of the sources.