It supports dual x8/x8 config. But the performance drop from the full x16/x16 is very minimal. So nothing to worry about there!
Infact all P67 LGA1155 support x8/x8 only.
If you look at the specifications for the ASUS P8P67
or the P8P67 LE
or the P8P67-M
that's not true. It supports CrossFireX only. There is no SLI support.
1 x PCIe 2.0 x16 (single at x16)
1 x PCIe 2.0 x16 * (max. at x4 mode [Black])
*The PCIEX16_2 slot shares bandwidth with the PCIEX1_1 slot, PCIEX1_2 slot and USB3_34 connector. The PCIEX16_2 runs at x1 mode by default for system resource optimization.
You have to check the motherboard's specifications so that you know what you will be getting.