It would be nice if you can provide the maker and model of your board.
Does your board have 3 PCIe x16 slots? If you do and you plugged it to the middle, then
it will run at x8 mode. If that is the case, plug it to the first PCIe x16 slot and see if it runs at full speed.
After doing some research on the
difference between the 680i and the 680i LT.
The difference between between these 2 boards is
the 680i uses the 590MCP where as the 680i LT uses the 570MCP.
The 590MCP lets you run 2 cards at full x16 speed.
The 570MCP will reduce the speed down to x8 if your running SLI.
And since your 7950 GX2 is a dual gpu card, maybe that's why it's running at x8
instead of x16.
680i LT is capable of using full 16x bandwidth for both in SLI. But thats besides the point because its ONLY using one slot. The other is on the actual card. It can be used on SLI with boards with only one PCI-E 16x slot.