for your 13.5, 2 teeth over your brushed is a good start. Last year I tested with a 7.5 for a little. And I used 15, 16, 17 and 18 teeth with the 7.5 depending on traction, track size and my mood. Ultimately, I settled for a 6.5 with 15/16 teeth. I have only used the 5.5 / 6.5 /7.5 with sintered rotors. I've used the 13.5 with both. The sintered is noticably quicker, even in the hands of the less experienced. (faster is not always better).
A 13.5 with Sintered rotor can easily pull 2 more teeth. On wide open tracks without much jumps, I've gone up to +4 teeth. But you have to be a really smooth driver (and I'm not).