The evidence is also there on the Metal & Crystal graphs, third one down again.
It also looks like a 1.5X boost, but it varies slightly because the extractors get built at slightly different times, meaning slightly more or less income.
But, the 1.5X bonus (& 2X for Unfair) is obvious on the third income chart! Look at the first couple readings, and it will jump right out. Your first income reading will be 10, while the Hard AI's is 15! And this ratio continues indefinitely, as long as every thing else remains equal. Whereas, on Normal AI, the readings are equal (both 10's, etc). Test the income on a small map with a Desert homeworld as a start (since it already maxed out & can't be upgraded) - like the "Melting Point" or "Point Blank" map.
After about 15 minutes, quit and go back to watch the game from his (the AI's) perspective. In particular, pay attention to what, and how much tech he was able to research in that time, (as well as what he buys elsewhere), compared to what you've researched. The AI is able to do much more!
Finally, the Unfair AI also handles the graphs in this same manner, and here we know that there is an 'unfair income bonus', because the changelog say so. So where does this bonus show up??? Again, here it is only apparent in the total accrued graphs (3rd one down), where it now clearly shows a 2 to 1 ratio (or 100% extra income)!
[P.S. Yes, I know that other charts/graphs, like the "credit income rate" don't reflect this reality, but I assume the developers left those that way so that we could make meaningful comparisons of our performance. And I agree with that decision, that this is the way it 'should' be, while the total accrued charts show the "real" bottom line.
Originally, I stumbled across this discovery, while I was trying to figure out what tech the Hard AI first researchs in the tech tree. In otherwords, when I was studying which tech was the most valuable. As an aside, I concluded that the AI selects its research topics almost randomly, offering very few clues on which to pick!]