I too have had the Korx wimp out several times, but at least in my game it seems pretty random as to if they'll be strong or weak. I would split it at about 40/60 in my games (40 being the liklihood that they'll dominate; 60 being that they'll suck). From what I've observed in my games, when the Korx suck, they REALLY SUCK. However, when the Korx don't suck, they seem to be undefeatable.
I know this sounds crazy because by the looks of things, at least for the majority of you guys, the Korx always play terrible. But in one of my games, I had the difficulty on Tough (not an MV game), and the Korx just dominated me. It was on a 9 opponent Large map, and at the end of Year 1 the Korx had researched up to the 4th level of miniaturization (Ultimate???) and had over half of the Beam Weapons line researched, plus a good chunk out of Diplomacy, too.
BTW - this game was on TA version 2.02.
After January of the 2nd Year, the Korx had gone and invaded the Thalan Empire(!!!), AND, they won the war in under 2 months! The Thalan, as we probably all know, are one of the quickest races to expand, and, in my game, they were ranked towards the top in Militay and Industry. I thought this was crazy, but then when the Altarian Resistance got attacked by the Drengin in April, the little screen that pops up for all of the races that come to the Altarian's aid because of Super Organizer had the Korx declaring war on the Drengin due to similiar ethical standings with the Altarians, who are almost always Good.
In the end, the Korx won the game on account of Military Conquest (yeah, I lost). It took them only 4 Years and 3 Months to win. That's pushing it even for me. Long story short, I think the Korx either will be terrible, or they will hit the jackpot on how they distribute their bonuses at the start of the game. You guys are pretty much right, though - the Korx do tend to suck more then most other races.