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are more powerful psykers than regular astartes librarians?

No.
They are more psykically attuned then regular space marines, and their librarians are better than space marine librarians, but a grey knight on his own is no where near the power level of a normal librarian. You can see this in the way the way they have to cast a single spell as a squad, not one individually

That is a game mechanic, which becomes obvious since in the game mechanics 1 GK can cast as well as 10. Whether the average GK is better than the average libby is up for debate.

Isn't it mentioned in the Grey Knight novel with Hyperion that they cast as a squad? The sergeant casts spells while the rest fuel it.

Grey Knights recruit the strongest psykers so they probably would be more power on average.