I have 8000+ books on my T1 SD and I have them in author collections
I have 1500+ on my Aura SD and have them in series collections with one collection for books not in series.
The T1 is not any slower than it was when it had 50 books on it, seems pretty fast actually and only crashes occasionally which it did with 50 books on it. Mostly crashes when deleting books. My 350 and 600 never crashed
and both had a lot of books.
The Aura is slow when opening the shelf list (takes almost a minute) but seems fine other than that.
Helen