Hello David, i'll try to help...
The main reason to upgrade from the gibson 57 were
boomy and muddy bass
not enough highs
not much dynamics
no 'airyness' i wanted my guitar to sounds more like a hollowbody than like a solid body.
I hesitated between the stormy mondays that Jorge choose, the mules, or the mississipi queen P90 and the manhattan P90. i asked for advices on the bare knuckle forum, full of helpful people and i finally decided to ask to the maker of the bare knuckle pickups for advices. The advices given were for either the stormy mondays or the manhattan. some people also advised the missisipi queen for a less subtle, and more cutting through sound.
I choosed the manhattan because wanted the highs and sparkle of a single coil, with the power of a humbucker. and i wanted to avoid the nasal sound of a humbucker. Haven't tested the stormy mondays. I wish it would have been possible to test several pickups and i think you can try them and send them back for exchange, but sending them back and forth to bare knuckle is expensive.
I have upgraded the neck pickup only and after installation, the first thing i noticed is that the sound is much sweeter, noble, than the sound coming from the 57 in the bridge, taking into account the fact that the bridge pickup is in the bridge, of course.
the sound is just more musical. the bass are now present and defined, precise, but not boomy. there is a slight boost in the mediums, which gives a little 'oomph' to the sound, which is very musical to my ears. Much more highs than with the classic 57.
The sound i was after was basically kurt, adam rogers, and lage lund. I find that naturally, it is very lage lund like, very precise and acoustic. With tone rolled off, i'm closer to adam's tone that i ever been. The closer you move the pickup to the strings, the closer you get to a subtle thing that
kurt has in its sound that i can't describe.
The dynamics are much better too. you can hear the subtleties in your attacks. and i would say the sound is 'spacier' in general
I used to have the Empress para EQ always on and now i barely turn it on, because i'm much closer to the 'ideal' sound with guitar->delay->amp (i still like to decrease the bass a bit, otherwise, flat EQ) only.
I have not upgraded the pots of anything else, just soldered the new pickup in place.
I think that the sounds samles in the bare knuckle site are quite accurate, listen to those of the mississipi queen cleans, then the manhattan cleans and you'll see what is the 'oomph' in mediums that i was mentioning. You can also hear the 'nasal' thing on the stormy monday, which is why i choosed P90. From my limited experiences, the samples seem accurate, so you might find them useful to choose the model too.
For me there is definately a noticable and efficient sonic improvement in upgrading from the gibson to the bare knuckle pickups. I have not tried the other models, but i think that even if there are subtle differences between the models, the overall quality should be better than mass produced pickups.
I am sure that i would be equally happy if i had chosed stormy mondays or maybe the missisipi queen, and tweaked the EQ a bit differently.
Anyway, hope that helps, don't hesitate to ask questions if you have some, maybe with a private message so we don't kill the original subject of the thread. You could also ask questions on the bare knuckle forum or ask the bare knuckle maker.
Hope that helps