So by this yes it might make it run x8 mode. It is just like if you doing a Sli or crossfire. It makes the x16 run at x8 mode. If you can I would put it in Pciex1
1 x PCI Express x16 slot, running at x4 (PCIEX4)
* The PCIEX4 slot shares bandwidth with the PCIEX8 and PCIEX16 slots. When the
PCIEX4 slot is populated, the PCIEX16 slot will operate at up to x8 mode and the
PCIEX8 will operate at up to x4 mode.
* When installing a x8 or above card in the PCIEX4 slot, make sure to set PCIE Slot
Configuration in BIOS Setup to x4. (Refer to Chapter 2, "BIOS Setup," "Peripherals,"
for more information.)