As far as JBs go, it's highly guitar specific.
A lot of people will tell you older JBs sound better, and if someone hasn't already said that, they will. How much of that is attributed to changing spec and how much of that is attributed to pickup aging and degaussing and mellowing is conjecture that depends on the person. Most pickups sound better with age.
As far as new JBs go, most of them suck. The gold covered ones in like, 2006-2012 Schecters are amazing though. I have no idea what is up with those. If it's just a good match between pickup and guitar, or if it's a slightly different OEM spec or what. I had one in like, a $300 Schecter I got on an Amazon Prime deal, and it was an awesome pickup.
That's sort of a grey area. Sometimes, OEM spec models, and the off the shelf aftermarket models of guitar centric parts have slightly different specs. Mesa's V30s are one of the more prominent examples. I have no idea if OEM JBs and the off shelf ones have any variation, but it's not something that's out of the question. In general, most pickup connoisseurs consider OEM pickups to be slightly less valuable. I have no idea if there is any reason to that or if it's elitism or whatever. But if I'm buying a used JB or whatever, I will almost always go for the one that was sold aftermarket in a clear box for $70-80 over the one someone pulled out of a guitar it was stock on. If they are actually different or not is conjecture, but that's just how the market works.
This guitar to be exact. The OEM JBs with covers in those decade old schecters sound awesome.
I actually know people who have pulled the gold covered ones from modern Schecters and sold the guitar but kept the pickup. I have no idea what is up with that. If it's just the same spec as all modern JBs or what, but they sound great in those.
SD's in general have a difference between F spaced pickups and non F spaced pickups. It varies from manufacturer to manufacturer how different they are. But SD has some models where the F spaced ones and the non F spaced ones are practically different pickups. There will obviously always be a difference, but some manufacturers have f spaced and non f spaced variations of the same pickup that sound practically identical, but SD has some models where people will be raving about the non F spaced one they've had forever and then get an F spaced one and be like, "this is total shit".
A lot of pickups are also scale length specific, which is something I think a lot of people don't consider. 24.75" guitars and 25.5" guitars have different harmonic content. In general, some of the more abrasive, obnoxious sounding pickups that don't work in a 25.5" guitar at all can sound great in a 24.75" one. It works the other way too. There are plenty of pickups that are perfectly fine in 25.5" guitars that are absolute trash in Gibson scales.