Technically, it should work. You can use 1x,4x,and 8x in a 16x slot, but not vice versa.   Regards, 
Hi Guys. I need help with this: I have a PERC H800, and it's a PCI-e 8X, and I wonder if I can use it in a slot PCI-e 16x in the other server that have only PCI-e 16x. Can i use this PERC H800 PCI-e 8x  on a PCI-e 16x slot? Thanks.
PERC H800 PCI-e 8x on a PCI-e 16x slot