A research done in Brazil (where racial quotas for universities were implemented) showed that students that got in through quotas performed better, on average, than "normal" students.
So I guess you are right.
In india the opposite is true. A significant fraction of student admitted through race quota fail and drop out. Now they want to reduce the exam difficulty for them!
Can you cite your sources? Br has been in the whole trend of positive discrimination for a long time and I doubt this study isn't biased as most of their society when it comes to everyone that isn't "white".