... Compare and contrast with, say, Pedro Delgado, Carlos Sastre or Alberto Contador - climbing specialists who were/are genuine GC contenders
Can't speak for Delgado (I don't have enough miles on the counter to have seen him riding) but I wouldn't put Sastre and Contador in the same list. I would actually put Sastre and Schleck in the same group, in my opinion they are both climbing specialists (who will both loose a lot of time in a TT).
But maybe, my problem is that I don't quite understand your definition of a GC contender. Since I would put Rodriguez and VDB also in the same group as Schleck. They (unfortunately) are a bit too weak in TT to be a (multiple-)TdF threat.
Personaly I would put Armstrong, Contador, Ullrich, Indurain, Wiggins,
Evans in a same category (you could make this list sooo much longer of course), which is "all-rounders" (the difference in their results are just depending on exactly how strong their climbing and/or TT is compared to their respective contendors of that time).
I would put people like earlier-mentioned-Sastre, Schleck, Pantani, Rodriguez ... as climbing specialists. Which doesn't mean they can't win the grand tours, but they just have to be climb very strong in the full tour, have the balls to attack from far away and (sometimes) have the brain to decide to focus on the Giro or Vuelta instead of the Tour de France.
And I guess the other way around will never work: people like Cancellara, Boardman, Martin will always loose too much time in the mountains.