I totally agree. We just finished our perf cycle here at the Goog, and in my self evaluation I rated my impact as "medium" on the various projects I worked on. (I started back in June, so I haven't been around very long, and I'm very much still ramping up.)

The peer evaluations I got all rated my impact as "important" to "critical". I don't feel like I did anything too outstanding; just doing what anybody would do, and I feel like I still have a tremendous amount to learn, and the senior personnel are still way ahead of me.

I definitely feel like a touch of impostor syndrome can be a help toward excelling, as long as it doesn't cripple you.