I'm not sure it's that simple, especially on large systems. I've seen several cases where an employee was responsible for only a small section of a huge system but was actually the only one who knew how the whole thing fit together.

We (his co-workers) all knew it, we were asking him questions all day. Our manager had a vague idea he was good, but that was it.

The system doesn't always reward the best performers...it doesn't even know who they are (that's why performance management is a multi-billion dollar industry).