There's a few production pickups that have extra magnets like those. The Gibson 500t has three oversized magnets with the 2 outer magnets looking similar to those:
https://www.lespaulforum.com/index.php?threads/dissecting-humbuckers-496r-500t.217526/
The Gibson Dirty Fingers, the Duncan Black Winter, and Duncan Invader use a three magnet set up as well.
You should expect a lot more low and high end response. A higher wind pickup will probably have enough midrange to keep up, but a low wind pickup might sound midscooped when you add extra magnets.
You can add spacer magnets on both sides, you just have to get the cable out of the way by sticking it out the side. The main differences I notice are a bit more power and the effects from the 2 types of alnicos if you chose to mix.
I don't know why Duncan and the other builders only do 3 double thick mags with ceramic, because it would work really well with alnico.
That's pretty cool. Would you say the central magnet, or the spacer makes a bigger difference in the sound? Or is there a 3rd variable? I've got a spare ceramic spacer that I could throw on a Custom 5. Would that bring it between a Custom 5 and a plain old Custom in sound?