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Yeah I think that all the cover is doing is allowing you to mount the P90 level with the strings, by reproducing the exact shape of the humbucker's mount.
The overall length of the Gibson mount HB is only 4mm longer than a soapbar P90, so some routing is very likely.
Not worth it for my guitar, will get some HB sized P90's or maybe restore it as is.
neu-
Normally with a P90 you have height adjustment of the whole pickup via the mounting screws. That’s what they were saying in that video, with the adaptor you lose that ability to set the overall pickup height (though you can still adjust the pole pieces).
Having googled it a bit more out of curiosity, I think the routing issue is to do with the corners needing to be routed out more for the P90s - that the shape is squarer and/or deeper at the corners?
I (obviously) have no hands-on experience though.
Regal
I think it's borderline and depends on how things have been routed. They are slightly different shapes and mount differently. Without a cover like that (which as they say in the video was 3D printed to exactly the right shape for that guitar) you would certainly see a gap around the edge of the P90.