ML110 G5 - PCIe 8x always running at 1x

[AJ Davey]

The excerpt from quickspecs below shows the ML110 G5 supports PCI-Express x8 at x8 in Slot 4.

Expansion Slots:
Slot 1: PCI 32-bit/33MHz at 3.3V
Slot 2: PCI-Express x8 connector with x1 link
Slot 3: PCI-Express x8 connector with x1 link
Slot 4: PCI-Express x8 connector with x8 link

Regardless of the type of card I try (Adaptec RAID, Intel RAID, QLogic FCHBA) it always reports the card as 8x supported 1x Selected.
I've even gone as far as testing an 8x to 16x expander I have and plugging in an old radeon graphics card, which then reports the card as 16x supported 1x selected.

How do I get Slot 4 to actually run at 8x?
It's not proving to be a problem with any of the cards tested, it's just annoying me.
Thanks,