You can freely explore and find anything you missed after finishing the story? That's a relief. Where did you hear this? I didn't see any mention in the reviews i read.
"Tomb Raider's adventure can be extended immensely by the sheer wealth of extra distractions on offer, and players are free to return to the island after the campaign's been defeated, to go for 100% completion."
Took me about 14 to finish the story, and I probably had enough collectables left over for another hour or two. Granted, this was a playtest and I had to play on Normal. If I got to choose I would have played Hard which would have probably added an hour or two.
... and Zeus screwed anything that moved. Or didn't move. His powers know no bounds.- Dahk1