I am happy to yeild to superior Skulltrail knowledge
I have been SLi for a few different chipsets and all have had 1 and 3 as x16. They also show SLi using 1 (x16) and 2 (x8) will go back to x8 SLi.
This is however for normal people without such mega hardware as Skulltrail.
I have however looked through all the documentation on Skulltrail provided by Intel and its crap for information. It gives no indication as to where you should place the cards for SLi or Xfire. So taking this to conclusion you could think you can use any of the slots and still get full power.