Good discussion. I haven't been around electric guitars long enough to have a strong opinion on Mustang pickups but here's my 2 cents...
I used to think the original pickups in my '65 Mustang were weak but it's all in how I coordinate my amp controls with my guitar controls. I'm sort of a purist anyway, and now I've discovered I like these original single coils.
When it comes to P90s, well again I don't have too much experience but my Epi Casino has the originals from '63 and this guitar wails. Lindy Frailin rewound the neck pup and the Woman Tone is creamy beautiful. I don't have the technical knowledge, but I do believe the people who rave about P90s.
So for hot rodding a RI Mustang, I'd be curious to hear a P90 at the neck.