I don't really think Dark Shadows/Gatsby comparison is right. GA were already let unimpressed by Depp in Pirates 4, and reaction for last Burton/Depp offering, AiW, was mixed. DS didn't seem all that exciting from the trailers too.
Gatsby OTOH stars Leo, fresh off Inception/Django mega popularity and the book is one of the most popular American classics. Plus, Baz movie trailers are always neat and stylish. Gatsby overperforming shouldn't be used as a gauge for IM3 WOM at all. Leo is on a roll and GG capitalized on that nicely.