Hi everyone.  It's been many years since my last build, so forgive my ignorance.  EVGA has always had great products and support, and a very knowledgeable community, so I think it may be time to jump back into the fray for a new build.
 
I've been looking over the X99 boards and 970/980 card specs, and I'm having a little confusion regarding compatibility.  The 900 series uses the PCI-E 3.0 bus, which should be backwards compatibile with 2.0 boards, where I'm assuming they would be seated in a x16 slot.  From what I can tell about PCI-E 3.0, this should mean that only a x8 slot on the 3.0 board is needed for these cards, and it looks like the X99 boards do have x8 slots that act as a mechanical x16 for this purpose.  So am I correct in that a 900 card can be seated in any of those slots?  (except the x4, of course)  If so, then what is the purpose of the dedicated x16 slots on the X99?
 
Thanks for any help in clearing this up.

That performance boost would mainly be seen in benching, not much difference at all for things like gaming.

One more thought:  If you populate certain slots, it may disable other things like an M2 or SATA express port as they may share bandwidth with PCI slots.