It's not 20 and it's not 2. It's not a person. It's a tool. It can make a person 100x more effective at certain specific things. It can make them 50% less effective at other things. I think, for most people and most things, it might be like a 25% performance boost, amortized over all (impactful) projects and time, but nobody can hope to quantify that with any degree of credibility yet.

  > but nobody can hope to quantify that with any degree of credibility yet

i'd like to think if it was really good, we would see product quality improve over time; iow less reported bugs, less support incidents, increased sign-ups etc, that could easily be quantified no?

Jevon's Paradox: more software will be produced, rather than fewer software engineers being employed.