I think it's more like 42 AWG on most P90's, but that's the measurement of the full diameter including shielding. The wire diameter itself can vary. 38AWG is sometimes used on CC pickups. A 5% wind variance really isn't noticeable...and pales in comparison to the affect the resistance and capacitance load has on the sound.
P90's are normally in the 7-8H inductance range. With 500k pots, ~100pF internal C and a typical ~400pF C guitar cable, the resonant peak will be in the 2-2.5kHz range. That gives it edge without being harsh. It's nothing magical. A ~2kHz peak works great for all kinds of things. An AlNiCo V magnet should generally be the most efficient. The lower power AlNiCo types decrease fundamental note strength and sensitivity to smaller vibrations. It's really the same affect you'd get raising the pole pieces and lowering the pickup height. AlNiCo V is just more versatile. A Ceramic 8 bar will likely saturate the core material, lowering inductance and possibly creating strong magnetic eddy currents which reduce the highs. That's what the tone knob is for, init? People might mistake it for more bass, but it's really just less efficient. If you get the resonant peak at ~2-2.2kHz, you probably won't feel the need to lower the tone knob in general, but it will sweeten and fatten a bit from 10-5 as the peak height & frequency move down a bit -- very simple and useful tones.
I have a bog standard GFS AlNiCo V P90 in the neck of my XV-585 with 500k pots. I use a very low ~160pF C cable. I wired a 330pF cap over the toggle switch lug and it sounds just gorgeous. I adjusted the pole screws and pickup height to where the low E string sounds "woody" and the high E sounds fat. I set the other pole screws for balance with the G and B poles well below the cover because of the pickup screw between them -- probably wouldn't need to if the pickup height screws were solid Nickel.