There should be no problem at all, and for your information there is no 2.1 slot at all. There is only 2.0 and 3.0. A 2.1 card will be fine in both 2.0 and 3.0. And I don't think there will be any performance drops.
Why would they make a PCIE 2.1 card if there is no PCIE 2.1 slot? Seems like it's just a way to make things confusing.