But do you find a single coil sized humbucker reacts in this way like a humbuker or like a single coil?
ev1ltwin - you're really selling me on a Warpig! I take it you don't find that when you move to leads the treble is drowned out by all that mid and bass.
Disclaimer: I'm going to ramble a bit and touch on different points... tl:dr style
First: A single-coil size humbucker can sound much more like a humbucker than a single-coil. A seymour duncan hot rails sounds like a high-output dark humbucker (with all of the muddiness too, but that's a different topic), and a Dimarzio pro-track sounds 95% like a PAF (to the point where it'd be damn near impossible to tell the difference unless you were listening for it). Single-coil sized humbuckers and full size are not the exact same because of the size/shape of the magnetic fields, but the coils are wound to compensate for it. It's pretty nifty imo.
Second: I think you're over-analyzing the "darkness" of the warpig. I think there's a lot of negative connotations when referring to something as dark that I think are unwarranted. Maybe a better way to describe it instead of dark is richer, fuller, and, most importantly, less sharp-sounding. Go to the BKP website, and check out the classic rock clip of the Abraxas. Then compare it to the Riff-Raff's classic rock clip. The Abraxas sounds darker than the riff-raff, but not in a bad way (the emg81 was much brighter than riff-raff btw). The warpig is still really clear in that you hear all the notes in the chord; it doesn't get muddy unlike other high-output pssive pickups from other manufacturers. Also, it's not like there's no treble in the pickup! It can shriek if you set your amp that way.
Third: My favorite lead tone is a bridge pickup tone which is dense, "chewy", and with good presence on the top-end. The closest I've found to that sound in my head is 3:15-3:35 of Metallica's "Leper Messiah":
http://www.youtube.com/watch?v=lI6Ubb7zJI8. What I've experienced is that it's easier to get the thickness from the pickup and brighten it up with the presence control on the amp. I found myself unsatisfied with the emg81 because it was thinner, sharper, and brighter. What I had to do was turn the presence down on the amp so my lead tone didn't sound harsh. With the warpig, I can get a sound that is denser with more presence at the same time.
Finally: All I can say is that I like my A-pig. I'm not saying that it can get you closer to what you want than a Nailbomb or Holydiver. I'm just not qualified to make that determination! In all honesty, I think you'll be happy with any of the 3, but from what Eric says, it sounds like you should decide between the Holydiver and the Warpig. Especially if you play with a lot of gain, the differences between pickups get less and less. Good luck with the decision.