Re: Good passive SD pickup for thrash metal?
Cheaper Alternative: Swap the magnet for an Alnico 8. People say positive things about the JB8. The only way I'd ever try a JB again is if it had an a8.
Duncan Distortion really isn't what you're looking for unless you're tuning down and using thick strings (I have one in a guitar tuned to B Standard, I wouldn't try to play old thrash with it). Try the PATB-2 for E/Eb.
Hetfield used the Duncan Invader for the rhythm tracks on some older Metallica albums (possibly Master of Puppets? opinions vary), but that won't help you unless you have a Mesa/Boogie MK2C++ head.
Alternative to the Dimebucker:
Bill Lawrence USA L500XL or Wilde Pickups L500XL. BLUSA is supposed to be more precise (ceramic mag), Wilde pickups are supposed to be a little warmer (alnicoV), BLUSA (I think) were used back in the heyday of thrash (Kerry King and Jeff Hanneman had them in their Warlock's and Les Paul's, respectively)