I personally don't see sticking some psyker's DNA into geneseed and somehow magically popping out a bunch of powerful librarians as particularly plausible (or even simply sticking human DNA into geneseed to begin with).
The Blood Ravens maintain a high number of powerful librarians, but that is because they specifically look for psykers during the recruitment process. Even Grey Knights need to already be psykers before being recruited for the implantation process.
The only geneseed that I can think of that actually seemed to induce psychic mutation is that of the Thousand Sons, and we know that their geneseed was extremely unstable.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.