How much difference does the cover make? I did a test with and without the cover, loaded and unloaded. Unloaded, the difference is nearly 3dB, but loaded the difference drops to only 1dB.
Technically, that 1dB audible difference should be audible, but just barely. Also note that the peak frequency increases with the cover removed, possibly due to a reduction in capacitance between the coils and the cover. I have tested the covers from the Seymour Duncan Seth Lover SH55 set and found that they are nearly identical.
I'm starting in on a series of humbucker testing. I don't have many points of comparison yet, so it's hard to say how hot or cool this set it relative to other popular sets. Subjectively, I like how they sound.
Copious amounts of wax:
No metal or wood spacers, just four plastic washers hold the magnets in place, and you can see that DiMarzio embeds extra slugs in between the poles and screws, probably to decrease the magnetic reluctance of the pickup by increasing the permeable mass within the core: