As someone who's owned several Phat Cats, I have to disagree with this. I was motivated to experiment with pairing different magnets into P-90's because my Phat Cats were such a disappointment, they don't sound anything like P-90's. Putting in A5's makes a world of difference, like taking a blanket off of a speaker. After trying various combinations, my favorite P-90 mag combo is an A8/A4 in the bridge and an A5/A4 in the neck. Big improvement in both slots.
Changing magnets in Phat Cats isn't as easy as with normal P-90's and P-90 size HB's, as Phat Cats are held together solely by the cover being soldered on, instead of the usual 2 screws in the baseplate. Not the best arrangement. You can only lift one side of the cover, as there's wires on the other side that can snap. Many P-90's these days also have an additional ground wire (3rd wire), unlike Phat Cats.
Forum winder Zhang said that he thinks the neck Phat Cat is wound too hot, so he took 1,000 turns of wire off of his to get more treble. Non-winders accomplish something similar by putting in brighter magnets. If you don't want to change magnets, it's simpler to get a GFS Mean 90, which has stock A5's. A lot of members think it's a very good sounding PU.