Re: squier pickup upgrade
The DiMarzio Area 58 in the middle and Area 61 or Injector Neck in the neck will pair very will with a JB, as will other combinations of DiMarzio's A2-based singles (54 Pro, Areas, Injector Neck).
If your primary intention with the middle pickup is to get outstanding quack in the 2 & 4 positions, an Area 67 will work well...even when paired with hotter pickups like the Injector Neck. Think of it as something like a 6k Fender.
If you prefer to have a fuller but bright sound in the middle slot then an Area 58 is a good choice. It's like a single coil wound to something like 7.3k
-An Area 61 is like a Texas Special, but just a tad bit fatter and just slightly darker. Adding a treble bleed will make the 61 will thin out and sound more like a vintage single when played clean. Under gain it will clean up nicely with some shimmer as you roll down the volume.
-An Injector is pretty fat with a strong midrange that is pretty smooth up to the low treble area. It's like a cross between a p90 and a fat single coil. It will be able to keep up with a JB when both are played with gain.
There are plenty of demos on youtube.