The older I get, the more I think that the best pickup for metal is the humbucker in your guitar. Hook it up to a metal pedal and have fun. If I were to build a guitar from scratch, I'd probably use something like a SD Screamin' Demon, and not just because I'm a Lynch fan. It's low (relatively) output and reasonably bright, more PAF-like. The cool thing with that is that if you want to play clean, you can still get a very nice clean sound, unlike the muddiness you'll get with highly-overwound passive pickups. For heavier stuff, let your metal pedal/Tubescreamer/amph take it from there. Even with a high-output humbucker, you can get a pretty nice clean sound by hooking up a push-pull to run the coils in parallel instead of series.
Having said all that, I thought at one point that I'd yank the JB out of my DK2M and put in a Screamin' Demon. I haven't found any reason to spend the money. The JB sounds great to me and if I want cleaner stuff I can switch to the Jazz in the neck or go middle position.
With my old Floyd Rose guitar, I spent a lot of money on SD and Dimarzio pickups (search here for all the gory details). Did they make an improvement? Yeah, but it was remarkably subtle. In retrospect, I think the reality was that I was just trying to find an excuse to like that guitar. I love my DK2M so I have no reason to mess with it, beyond the little trem stop (for string changes) and the treble bleed cap I added. The good news is that I can spend more time playing instead of messing with the guitar itself.