My conversations with Google reps and experience at an agency differ from this. My experience has been and Google reps have confirmed to me that a performant enough site will not benefit from enabling amp.

It seems like they do assume amp pages are high-performance, but they're not the only way to achieve high performance.

Any sources or data you can share on that?

Aren't AMP results above the traditional search results on mobile?

Yes, AMP alone will not improve your organic search results but why should you care when those results come after AMP.