However I would appreciate if you published an official sheet with all multipliers of difficulty levels. There are a lot of speculations here and there, but nothing seems to be clear.
Its easy to figure out. You can determine the amount of cheating by looking at the third charts down (for credits/metal/crystal). To get a quick picture of how much each AI is cheating: start a large game with each type of AI, quit after a few seconds or minutes, and look at the 3rd chart for credits "from" planet, etc. You will see 5 sloping lines. (Divide Hard/Normal to calculate, etc.)
For the DLC patch, cheats for Hard AIs were increased to 2.25 times bonuses. So yes, it is in fact double Goafan! The Hard AI now gets more than double our income, 2.25 times more credits (and metal & crystal) from planet income than you or I. Unfair gets 3.5 times.
Prior to the last patch it was 1.7 and 3.0, for Hard and Unfair. And back in the days of Entrenchment, Hard only got 1.5X's more.
(Cruel & Vicious also get speed bonuses on research times.)
I agree that the gap between Hard and Unfair is currently too large. But I also feel that Hard cheats too much, and that Cruel (or Vicious) are just ridiculous. I would like to see ALL of the cheats lowered to more reasonable amounts.
See some of my very old posts for more discussion of AI cheating.