Negligible up to a certain resolution, at least. Just most people don't have monitors at that high of a resolution without already investing in a mainboard that gives the best multi-GPU performance (duo-, tri- or quad-x16 PCIe).
The problem with 1156 is that the number of lanes available is limited by the on-die graphics controller. For most people who only use a single discrete card at most (or the onboard GPU at the least), this is not an issue. For enthusiasts who like to hook up mutiple GPUs, this does put a limit as to how much performance they will get out of these boards.
Be aware that sandy bridge will ultimately have the same limitations, so 1366 will still be the enthusiast performance platform of choice in the near future, and 1155 will be the economy and business class for a majority of the Intel builds.
I think all LGA 1366 processors, except the hexacores, have direct counterparts in the LGA 1156 line up.. Also, except a few extra features, the performance across the board is also similar be it for gaming or other works..