Not sure what exactly the difference in the KK 2nd Gen is, from the 1st, but I've used the 1st, and still have some. Very nice iron shafts, maybe the best I've used. I had the R, which may have been just a bit too much for my slow swing, but should be great for those that swing faster and using R. Have never used Xcaliber, so cannot make a comparison. I will say the KK shafts are the most consistent shafts I've ever found, as far as weights, all were exactly 71 gr., and I've had 10 of them. They also spined with the logo up, every one of them. And, the price is more than right.....although, out of R flex.
http://golfdealsandsteals.com/mitsubishi-rayon-kurokage-iron-shaft-set/