I've got a partsblaster P-Bass, which has a Squier body, a MIM neck, Badass bridge, Fender tuners & a Hopshot eXtender, Fender USA pickups, and my wiring harness (CTS/ Switchcraft/ Sprague).
It sounds and plays really well, a proper Precision grind!, and I'd have to see something very special to replace it!
I had it at the last SE Bass Bash.
In general, if I were looking to upgrade a P, I'd be looking for a set of US pickups, or maybe trying the Geezer ones.