It is fully backward compaitable.you will never lose performance even if you plug pci 3.0 card in 1.0.pci exp card never utilizes its speed.
meaning? is it like USB? just different versions for different speeds that's all? otherwise it will just run fine except that the speeds might be slower. but the question is how much slower? will it be noticable?
if I plugin a pcie3.0 gpu in pcie2.0, is it similar to plugging in a usb3 device into usb2 port?