Waking up an old thread.
Reversed P on a PJ is the way to go. A mystery why it hasn't been used more. The earlier Yamaha BBs, as someone mentioned, has it and they have a great slap sound. I've never come to terms with the regular PJ sound when using both pickups full. Especially when slapping the D-G strings sound thin and plastic, they just don't match and there is much less body in the tone. A reversed P- pickup in the PJ combination makes so much more sense since it beef up the D-G string and makes the E-A strings less boomy and more articulate. If you don't hear a significant difference I really can't understand how you're listening to bass sounds, it's so obvious.
On the other, only thinking about the solo'ed P-pickup, that's another issue. The traditional p-bass sound is loved and used so much because of it's character based on the way the E-A strings differ from the D-G and if you change that, you change bass history Only when using the PJ configuration and using both pickups fully is when the benefits from a reversed P-pickup shines.