After many years of no updates to the 7.6 Planning and Performance Tuning Guide, Veritas has now published a new guide for NetBackup 9.0, 220.127.116.11, 8.3.
Hopefully this is a 'work in progress', as it only covers the master server.
To @Marianne's point, yes, this is a work in progress. We started it last year and realized there was no practical way to tackle the entire document in one shot, so we are taking it in chunks (the best way to eat an elephant).
As part of the field team working on this, I invite you to provide feedback. If you have comments, needs that haven't been met by the past iterations of this guide you'd like inserted, or other elements that need coverage, feel free to bring them up and I'll do what I can to bring them back to the team. This requires a very specialized set of skills and knowledge, and so the team is not huge working on it, but we'll get things brought in as quickly as possible.