Am I right to assume that an NPC with the Sorcerer and Master Sorcerer Talent and a Psy Rating of 6 would all in all have 8 dice when manifesting a psychic power?

Yes you are.

And would probably get a grand total of two powers off before getting possessed by an unthinkable horror.

They wouldn't necessarily need to use all of these dice(as far as I remember), so they might not suffer more risks than the "average" Psi 1 Psyker

But is a Psyker with Psi Rating 6 and Master Sorcerer talent considered to be Psi Rating 8 in all respects, or does he only get to roll 8 dice in case he wants to destroy himself quickly with Perils of the Warp?

I am asking this because I am wondering whether he gets half of his WP bonus in Psychic & Minor Psychic Powers, once by raising his Psi Rating to 7 and again by raising it to 8, or not...

Since it's an NPC you could give him what ever you want too, in my opinion I would treat it like another two Levels of Psi-rating, but I can't find it anywhere at the moment, so it's just guessing

Jan Solo said:

Well he will get minor or major Arcana equal to three times his WP bonus when he selects the Sorcerer Talent, minor Arcana cost one, and major Arcana costs two. It works in a different way than gaining a Psy rating. It only has an effect in the sense that it confers a bonus to the Sorcerer Psykers Focus Power Action.

Don't forget to add +2 to power threshold and +20 to Psychic Phenomenon.

No it works differently when its a psyker. The +20 is still in effect. But the +2 to Thresholds doesn't apply when its a psykers that employs Arcana.