Nope you'll be fine.
I don't know the specific differences between PCI-e 2.0 and PCI-e 2.1 (most likely bandwidth). But a PCI-e 2.1 card is compatible with a PCI-e 2.0 slot. A PCI-e 1.x slot... well that's a different topic.
ok so the performance won't go down with the 2.1 card on a 2.0 slot