I have a Custom P-90 in the bridge of a Les Paul Studio. I love it. That said - I put that there because I wanted RAWK from that position. Not rock and roll, not rock, but RAWK. It delivers! I find the EQ description Duncan gives pretty accurate. Definitely pushing mids. and the highs are not harsh.
I can't speak to the hot P-90.
My custom has balls, punch, and bite though, in all the best ways. And it is definitely loud.