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.
Not sure why when Shields came over has any bearing on the point that fighters from other orgs have not done as well in the UFC. I really think/hope DC will do well, but I am going to wait to see how his first fight goes before I jump on the bandwagon. I think Mir will be a good test. He handled Barnett easily and if he does the same to Mir then he is going to make a big impact in the UFC I think.