I wouldn't say their ex-champs, Overeem, Diaz and Shields, have fared very well. None of them have a winning record. Bigfoot does, but that is about to go back to even soon. Cung Le has a winning record, and if he was in his prime he might have been a real force when he came over.
The guys that came over are good fighters but none of them are exceptional or are going to be champ IMO.
edit: I think DC might be the guy though.
Shields came over long ago.
Diaz beat the crap out of Penn and went toe to toe with Condit who is still a top 3 WW. How many people truly thought Diaz would beat GSP a top 3 P4P? Mir is not GSP.
Overeem has been iffy. Hendo has been solid. Many non champs have done well. Like Robbie Lawler and Woodley. Many are yet to fight like Jacre, Melendez, Rockhold, Feiajo...so it is hard to say yet.
But no one expected Diaz, Hendo, and other champs to come over and beat the UFC champs. Very few people picked Diaz. And more people picked Machida to beat Hendo, and Machid isn't even the champ.
Mir just doesn't match up well vs. DC. DC is also very composed, and won't easily sway from his gameplan and play Mir's game.