If a freeway is 16 lanes wide, but you have to get 8 cars down it, do the cars go twice as fast?
The 16/8/4/1x refers to the number of physical "lanes" (wires from the slot to the CPU) the slot has. The Generation (2 and 3 are all you need to worry about) refer to how much bandwidth each lane has, with Gen3 being double Gen2.
Having more bandwidth wont result in an increase to performance, unless your upgrading from a situation where you were already limited.
Or if I take your question a different way, yes you will have a rough doubling (often its an ~80-90% increase, drivers and games aren't perfect) of performance since you are now running two GPU's.
ok thanks for that info! and clearing up what the x8 or x16 means! And yes by performance i meant will i get a ~200% fps increase with CF on games that are compatible with it.