There are 8k backstabs and then there are 10k backstabs. I've never seen any shadow hit a 10k backstab, ever. I have, however, seen double projects be counted as one hit.
Myself and a guildie were messing around in outlaws den yesterday trying to see what the biggest hit we could get on each other without armor +wz expertise buff. In a 0/24/17 build I hit him for 4672 on a non crit SS (we tried 3 or 4 times but I never got a crit), based upon my 78% surge and the 30% crit dmg from circling shadows I would have hit him for 10810. A 10k hit for a shadow would be possible, however it would most likely be a setup, or the 1 in 1000 chance that the dude is an idiot and decided to queue for warzones in lvl 32 greens.