I'm not really sure how that works. You can be either a Psyker or a Sorcerer? Isn't Magnus know for his immense psyker powers and for being a sorcerer? Or I'm missing something?
There is a distinct difference in that in the lore ad novels almost anyone has the ability to become a Sorcerer though not many have the wits ti match that ambition, Pyksers from what I've learnt of it are more born with their powers unlike a their darker Counterparts that learn the lore.
I believe the explanation Ruven in the Night Lord trilogy on the topic of Navigators is similar to this, that one possess to lore to use it, the other was born to.
Psykers can become more powerful by becoming Sorcerers but at that point they stop using some of the powers they originally had in favor of what they gain.
Like Ahriman's power was originally in divination. Yet at then end of Prospero burns he discovers they have all gained all the regular powers of Astartes Psykers like when he tried to help one of his fellows and accidentally shot him with lightning and then realised that for some reason his words held power.
It is not that can't use them anymore, it is that they are too enraptured with what is new that they do not, or if they do so, very rarely.
I decided to represent this by cutting off what Psyker powers could be learnt once Shirou becomes a fully fledged Sorcerer, simply because there would e so much more to learn. He can still use what Psychic powers he has just that there will be better (read: more powerful and easier used) options available to him.
As for Magnus he is a difficult case as he already new some of the secrets in the warp. He was a powerful Psyker second only to the Emperor himself, yet both of them could be counted as Sorcerers due to how they gained such lore and knowledge bar the fact that at that point at least both of them didn't worship Chaos. It is unknown if Magnus does truly worship as with his opinion to Tzeentch is similar to his former Chief Librarian's. They hate the changer of ways, abeit for slightly different reasons