Firstly Zondarion. Read the fluff. Dreadnoughts are often brought out of their long sleeps between battles to give sage advice regarding battle tactics etcetera and have their great intellects intact in loyalist chapters. They aren't brain dead organically infused robots, those are servitors thank you good sir. They arent run by the instincts of the soldier, they are rational, often intelligent, great warriors, it is the way they are treated in most chaos chapters that drives them insane. Read the damn codex.
Secondly. Just because specific psycher dreadnoughts have not been mentioned in codexes does not mean they don't exist, it is only recently that a chaplain dreadnought has been done in forgeworld, and games workshop has many holes yet to fill in this rich and expansive game, hence the reason one can invent armies and chapters etc, because the universe is so damn big and there is so much to say regarding it.
Thirdly. The Thousand Sons being intelligent, generally rational, and respectful of knowledge would not just treat a fellow fallen sorcerer like a piece of machinery to waste away into madness as just another butchery machine like the less refined chapters. They would see a fantastic mind that must be preserved and would thus treat it with due reverence, maintaining him, providing him with the texts he requires to view through his new autosences, and properly infusing his sarcophagus with the appropriate psychically resonant materials (think psychic hood) etcetera. Giving him the same reverance as a loyalist marine might for a fallen hero of his kin. And thus being a rational, strong minded sorcerer still, will have not fallen victim to the curse. (and may very well have been interred into the dreadnought long after the curse).
Fourthly. Eldar vehicles have had certain shields, and rules have been released giving variouse ork and imperial vehicles crude shields that provide invulnerable saves. Enough said.
Fifthly. I explicitly stated that the dreadnought would be given a leadership value of 10, as of course it doesn't already have one. Have you ever heard of an exception?? And to follow that I also stated he would not suffer from perils of the warp, so again you have demonstrated your inept reading skills to match your obviously extensive knowledge of fluff, as well as the nature of the game itself. I will expand on this point however, the need not to have to take perils of the warp tests is due to this sorcerers obvious blessings given to him by tzeench as well as the multitude of wards and protective incantations worked into his armor and sarcophagus.
Sixthly. There is in fact a great difference between a gibbering insane angry tortured prisoner of a dreadnought that risks going firefrenzy every now and then and a mindless automaton that has been reduced to ash and not only requires direction to move, but also is not PHYSICALLY PRESENT and thus not subject to being affected by minor damage that causes stun and shaken results so therefore even the basic tzeench dreadnought would act quite differently than the standard chaos dreadnought.
Zondarion. I give a bonus provided purely for your benefit, a friendly series of words containing a rich fountain of knowledge that will aid you in your limitless journey through life. Don't... and I say it again to prevent confusion... DO NOT open your mouth (or hit your keyboard) unless you have anything intelligent worth saying, and have thought of it for great length beforehand.
*Cleaning blood off axe cyclopse*