I have been watching Borussia games almost every week this season, to be frank. And I really doubt he would add any significant increase of quality over Bonucci. He is REALLY mistake prone defender, he used to have really bad games, with a lot of easy errors, missed passes and risky tackles. Bonucci and Hummels have the same problem - concentration. That's why I believe it's just not worth a shot to exchange Bonucci for Hummels, especialliy if we had to pay some crazy money for Mats.
If you don't believe me, check my posts. I write a lot about Hummels and Borussia on tuz.