Have you considered looking at the Blackouts?
I'm thinking an active pickup is going to accomodate a metal tone in a JS moreso than a passive will. To my ears most active pickups sound similar regardless of what guitar they're in, so consider that option. EMG's, Blackouts, etc are namely geared towards metal so keep that in mind.
You can throw a JB into your guitar, but there's a very likely chance that in the end it's not going to be the necessary tone your after, simply because Dave's guitar at the time likely was nothing like a Jag Stang. It just seems like for a guitar such as the JS, a set of EMG's or Blackouts will probably get you closer to the tone you're after as opposed to passive.
