I think the Korath and Drengin manufacturing techs need a tweak in ToA.

First, I think there are just too many +% manufacturing buildings for these civs. The Drengin can get up to an 80% manufacturing bonus from four buildings (Devil's Forge, Enhanced Slavelings, Artificial Slavelings, Death Furnaces, each 20%). The Korath add two more that could raise their manufacturing bonus to an insane 130% (Wretched Harvester, Aul Incinerator).

These are impressive bonuses, but they only come into play on very large planets where you can build many Slave Canyons or on smaller planets with an impressive manufacturing bonus tile. For most planets, this wide selection of +% buildings is just not practical or even useable. And although you get +20% military production for researching the Slaveling techs, they are generally not worth the investment.

If only these top tier buildings added to base manufacturing and gave a +% bonus, they would be ideal upgrades from the Slave Canyon/Devil's Forge. At the very least, they should be cheaper than the Devil's Forge, which would then make them ideal replacements. Right now they cost 5BC, while the Forge costs only 4BC. 

Second, I have a real problem with the Korath Aul Incinerator. Its +30% manufacturing bonus is never worth the sacrifice of a planetary tile, exactly because of the number of +% buildings I've listed above. For example, if you built a Devil's forge and a Death Furnace on two tiles you will get a +40% bonus. The Incinerator gives you 30% for those same two tiles. There may be a limited situation on a PQ26 where you could build all six +% buildings and have a tile to spare. But how often does that happen?  Worse - you won't save any money with the Incinerator, because it costs 5BC. Worst -  you might lower the morale of the planet by removing a tile.

I think it would be worth the sacrifice of a planetary tile if the Incinerator were free and it gave some other bonus - maybe an economic or morale bonus. As it is, I would never build one.