Re: wire insulation. why does it matter
So... hypothetically speaking a 1 micron thick formvar shouldnt be different than a 1 micron plain enamel...
In theory, yes, but the insulation differs in how it's been distributed on the wire, and that's the biggest factor that will affect what's called "coil geometry". I don't have the necessary scientific background to explain it and motivate it, mind you, but it just happens and what's documented on the different winders R&D processes.
What I think is that in the beginning of coil winding applied to guitar p'ups, every manufacturer basically used the kind of wire they could get from the supplier, so wire was wire, just like a p'up was a p'up.
I haven't seen all the different insulation under the microscope, but seeing at Plain Enamel, it looks a little bit like barb wire, and this helps with the coil keeping the shape it was wound in the first place without altering the coil geometry with time and without even using a lot of tension, allowing to wind "looser" coils without the need of potting to prevent squealing.
Using poly nylon insulation, the surface of the wire is the smoothest, so if not enough tension is applied to the wiring, squealing is almost inevitable, so potting is a necessity, being the lesser of two evils. Fortunately, due to the dielectric properties making the wire "brighter", potting doesn't alter the tone in a bad way, and can even "enhance" the final outcome.
The winder can take all this characteristics into count when designing a p'up to perform a certain task, so no method is inherently better than the other, just different and/or cheaper/more expensive, more/less traditional.
In a nutshell, one can't really say potted vs. unpotted one is "better" than the other, as it all boils down to the intended design. The same with "coil mismatch". Historically, PAF coils were all mismatched only because they were wound carelessly with no quality control whatsoever. Hype made a fault to a feature. Also, the PAF p'up sold was NOT even the same as the patent applied by Gibson shows, and that the final outcome was "produced by commitee", and the complete disregard to the required specs by using whatever part was the cheapest, specially regarding magnet type. As long as it was "Alnico" written on the invoice, it was all good.
HTH,