I have been playing the Korath a lot in this beta, and the amount of Pop Growth bonus that the Korath can accumulate early in the game may be a bit too much.
The Korath can generate a 50% increase in Pop Growth with only about 10 technologies early in the game. This bonus allow them to expand very quickly because their starter colony tax bases grows very fast. They never actually get into a money crunch. Their slave based factories are also very cheap in maintenance.
The technologies that apply:
Corrupted Genetics + 10 Moral +10 Pop Growth + 10 Loyalty - This is the cheapest tech for the Korgath and leads into
Xeno Medicine + 10 Pop Growth
Xeno Farm I+II+III - Each of these add +10 Pop Growth to the Korath. Xeno Farming 1 is important enough to research it fairly early. I find it strange that the Korath get +10 here while other races who also have Xeno Farming don't get +10 Pop Growth.
Conquest Strategies - "Recruitment Center" - Recruitment Center gives a planet 20% population growth and 10% Economy.
Together that is 50% Pop Growth. This causes planets to be a lot more self-sufficient early on then other races. A home world and/or 1 or 2 industry worlds can churn out colony ships at a rate that will dominate other races.
The Torians are supposed to be the Pop Growth race, but they get a significant production block early on by only being able to build that 1 deep core mine on each planet. Even then I do not thing they get as many Pop Growth bonuses as the Korath get, not to mention that the +10 Moral Bonus helps tremendously as well.
Korath are limited with Diplomacy throughout the game sure, but they are evil so that doesn't hamper them at all. The Influence problems in the mid game can slow them down, but the fact they usually out colonized the other races by a wide margin means they are still at least in the Top 3 in influence.
Solution ideas:
One possibility is to remove the Pop Growth bonus. Still that would be boring. I think keeping the Growth but adding either more maintenance to the slave factories, or adding a negative bonus to them that reduced Pop Growth for the Korath instead of influence or maybe both.