There should be no performance drop at all. Even the new cards these days don't use PCIe 2.0 x16 to its highest capacity.
PCIe 3.0 is just like USB 3.0 was. It's just a marketing thing to say "We have something new", but you really won't see any real world benefits other than being able to say you have a PCIe 3.0 card.
For external drive transfers USB 3.0 was pretty huge
There should be no performance drop at all. Even the new cards these days don't use PCIe 2.0 x16 to its highest capacity.
PCIe 3.0 is just like USB 3.0 was. It's just a marketing thing to say "We have something new", but you really won't see any real world benefits other than being able to say you have a PCIe 3.0 card.