Well, the Korx just beat me! Huge galaxy, tight clusters, everything common, masochistic, with the Drengin, Korath, Krynn and Terrans also playing, and me as Thalans. I resigned the game after they drove me back to Thala and a few barren worlds - at this point, they had 47 colonies to my 6, with the Terrans and Korath next with about 20 each. They were at war with me, the Drengin and the Krynn and beating us all. Out-expanded, out-researched and out-built everyone, with a strong economy, and conducted invasions well, conquering my best planets in fairly short order. Built Galactic Stock Exchange, Restaurant, Bazaar, Xinathium and the Mercenary Academy. In short, entirely unlike they have ever played before.
What went right? Well, I was stuck out on a limb (Thala and two other worlds literally half the galaxy from everything else) while they were sharing a big cluster with the Krynn and Korath, so I wasn't going to be much of an impediment, and they had the pathetic Drengin as a buffer between them and the Terrans. Friendship with the Korath seems to have helped, there was obviously a bit of tech trading between the two as some of my modded Korath research buildings were on their planets along with Devil's Forges, and they had traded for cut-throat trading too - but I should point out that they weren't relying on the Korath for military help, the Korx ships were slightly more advanced, using medium hulls while the Korath stuck to small. The one thing they weren't doing was building factories - in fact I'm not sure how they were churning so many ships out with such a small industrial base.
I was playing with my hybrid mod, based on MarvinKosh's & Tolmekian's and the commentary in the threads for those two mods, along with my own Drengin and Korath changes (of which the Korath have been far more effective). So the majority of tech tree issues have been fixed, but I can't imagine that alone made the difference otherwise you would have noticed it already in your games too.
I'm going to play through a game as the Drengin to see if I can work out what's making them suck so much, I'll throw the Korx into the mix and see how they do!