If you ask me, Mustaine isn't even CLOSE to thin-sounding. His tone is metal defined. It's at least a lot better than Metallica's EMG-powered clankiness or Dimebag's yelping, squealing lead tones. You're right about one thing for sure, though.... pickups are only PART of the equation.
For the record, I get really solid low-end from JBs. I don't know how heavy your power chords possibly need to be, but JBs work for my version of metal tone.
TwilightOdyssey - I agree that Screamin' Demons are awesome pickups. I have one in my Strat right now. I don't know if I would call it a metal pickup, but you could if that was the tone you wanted from your metal. The only reason I didn't mention it is because I use it mainly for hard rock, blues and punk.... It's an awesome pickup, though and very very versatile. It's a lot warmer and dirtier than the name lets on.
EDIT: Ranalli - I'm not trying to start an argument... I'm just curious.... What's your choice for versatile metal pickup?