I've never used Phat Cats, but with P90's and HB sized P90's, I also prefer 500K volume and 250K tone. I also generally like a smaller value tone cap: anywhere from 0.005uF to 0.015uF, depending on how bright the guitar is.
But then, I tend to ride the tone control a lot.
If you're a "set it and forget it" player, you'll have to experiment with values and see what you like.