We have done Kune X, and Robert is exactly right, you get the hybrid vigour - faster growth, as well as a larger pig than a straight Kune. We sold ours on before finishing, so cannot comment on the meat itself, but would fully expect it to be leaner than KK alone.
On fattening boars in general, we have a fattening group which we chuck a coupe of weaners into each month, and take off a couple of finishers, giving a rolling production line. Most of the time these are entire males, and we just chuck the eight week olds in. If from different litters we mix then first, so at least a friendly pair is going in. However apart from the odd scratch, there have been no issues with doing this, and that includes putting Mangalitza crosses in.
I have not had hands on experience with Kune boars, and Roberts experience and knowldege that they are more agressive is interesting.