Yes, I may seem too oversimplifying - indeed there are differences. If you really skimp on wire gauge etc. it may have a clear difference. But in case on generally well build pickups, say between DiMarzio and Seymour or even OEM pickups, the differences are very very minute so as to not IMO be worth changing from one to another really. Even if there are differences, I think more often than not in pickup changes and happiness with your pickup has more to do with the placebo effect than anything else.
I'm pretty sure if I had someone ask me to change some nice high-output Epiphone pickups to similar Seymours or whatever, and instead I'd just change the plastic top parts, the guy would not be able to tell the difference and would swear they sound way better now
I used to have a no-name partscaster in the store I worked in, which I sometimes gave customers to try for comparison on how a partscaster can be, and people liked fine when they tried it, but suggested changing the (no-name bucker) pickup for better usually. I started telling people it has a PRS pickup in it _before_ they plugged it in, and whenever I did that people only commented how good the pickup sounded, never got any suggestions to change. I never even knew what brand it actually was. I did a lot of similar evil tests back then, like telling someone he's playing an SS amp when it was tube and lo behold it does sound 'kinda metallic' to him, and vice versa
and then I go 'oh wait it IS tube' and the customer is puzzled and 'lemme try that again, well yeah, it isn't that bad' LOL.
Meaning, if you're unhappy with the pickups you have, it may be worth a change if you're going for entirely different output level or something, but not to another quite similar pickup; it's cheaper and better just to adjust the EQ rather or pole heights or pickup height etc. Much more change can be had with those than changing a pickup. Can be a different story with some farestern super cheap copy pickups - those can indeed vary a lot how they sound, but I'm talking regular OK quality pickups.
If you put a Seymour like JB or Jazz or PAF or whatever that have the same output in the same guitar, they all sound just the same to me. Have someone play you back the sound samples of similar PU's from the Seymour site for instance, I'd be more than amazed if you could tell them apart in a blind test.
There's a LOT of bull and superstition about guitars going round, and most of it is actually based on factual things too and at least partially true, but, by and large, most this kind of things tend to get exaggerated to the point of ridiculous with all kinds of 'mojo' issues...that's why I try to suggest otherwise when I can, just to keep a kind of reality check around.
