Re: CAn I swap out P100's for P90s in my LP SPecial?
When folks talk about the rule of "250k for single coils", it really means Fender single coils.
P90's are a different animal, much beefier, especially when they're over 9k, so you can use 500k pots, although this is really a matter of personal taste. If your guitar is unusually bright and/or you want a smoother/rounder tone, 250-300k pots would be fine.
FWIW, I tend to prefer 250-300k pots with both P90's and humbuckers, unless the guitar's natural tone is unusually dark.