Re: what is the story with Mini Humbuckers?
Epiphone made a whole bunch of pickup designs in the late 40's/early 50's for their archtops and finally settled on something called the 'NY' pickup; it looks like a mini HB in size, but was a single coil. When Gibson bought Epiphone in the mid-50's, they got all the guitars in stock and all the parts, so they kept building them the same way. They used the old Epi parts up till the early 60's, then developed the mini HB to fit the Epi body templates (ex. Sheraton, Crestwood, Broadway models). They also used it in the Johnny Smith archtop and redesigned it for the Firebirds (basically two Melody Maker coils stuck under a cover). Since the earliest LP reissues in the late 60's were routed for P90's, they eventually installed minis in later models (Deluxes) since they fit the bodies. The Deluxes sold pretty well, so they kept it that way for awhile.
Back in the day, the guitar companies were building for jazz and country players and high output was not a consideration (they discontinued the Alnico 5 P90 because it was too hot). A clear warm tone was what people wanted, and the mini works great for that sound. No surprise if it doesn't work well for a rock sound, neither does an L5.