I actually don't think the middle to late albums are all that well mastered. Sure they're not pop, but I think they suffered from the same mastering mentality of "louder is better" as the rest of the mainstream music scene has.

For comparison's sake, I think the newest Coldplay album "A Rush of Blood to the Head" sounds waaaay better from a mastering point of view than those Metallica CDs, but it has definitely received the radio treatment.