The CPU has 16 lanes of PCIe 2.0 for a total data rate bandwidth of 16GB/sec.
PCIe 2.0 lane by definition has a transfer rate of 5.0GT/sec and runs at a speed of 5.0 Ghz.
The P67 chipset has 5GB/sec of data rate bandwidth x 1.
P67's chipset PCIe lanes operate at a full PCIexpress 2.0 transfer rate speed.
P55 chipset diagram.
The CPU has 16 lanes of PCIe 2.0 for a total data rate bandwidth of 16GB/sec (so 1156 and 1155 CPUs don't differ here).
However, on the P55 chipset, each PCIe lane only offer signaling/transfer rates up to 2.5GT/s (which means they probably operate at 2.5 Ghz only), which is the same speed as PCI Express 1.0 (gen one).
The P55 chipset has 0.5GB/sec data rate bandwidth x 1.
P55's chipset PCIe lanes operate at PCIexpress 1.0 transfer rate speeds.
Low to mid-range P55 boards would run dual graphics cards in 16x/4x such that the first PCI Express slot wqould use the lanes from the CPU and the second PCI Express slot would use the lanes from the P55 chipset.
P67 chipset offers 2x the transfer rate over P55. Therefore, what was PCIe x4 on P55 is essentially as fast as PCIe x8 on P55 from a transfer rate perspective. However, this is still called PCIe x4 on P67 but the transfer rate is twice as fast as it was on P55 under the same x4 lanes.
This explain why P67 x4 is not bottlenecking the HD6950, I wouldn't worry about really you probably going to lose 4-5 percent performance depending on the game you play.