I updated OctoWS2811 for Teensy 3.5 and 3.6. I tested with 1920 LEDs.
It certainly should work with several thousand. As far as OctoWS2811 is concerned, these new boards are pretty much the same as Teensy 3.2. Of course, the CPU is faster, so you can do more to compute whatever animation you're doing.
All WS2812 type LEDs take 30 us per LED to update, so the length of your strips determines the max update speed. Teensy 3.2, 3.5, 3.6 will all update the strips at the same speed, because the communication protocol is the limiting factor.
In theory, these new boards ought to be able to update a pretty incredible number of LEDs, if you're willing to accept the slower update rate due to longer strips.