Hmmm, I feel like the opposite is true. Take for example Ivy Bridge CPUs, which the 6,1 uses:
- The fastest 12-core CPU runs at 2.7GHz
- The fastest 6-core CPU runs at 3.7GHz
So a computer with two sockets could run two hexacore CPUs for a total of 12-cores at 3.7Ghz. A Mac Pro or any other computer with one socket can also run 12-cores, but they will be stuck at 2.7GHz. The 2-CPU setup with the same number of cores is literally 1GHz faster.
Also, two socket motherboards support many more RAM slots. So if you have very high RAM needs, the 2-CPU setup is potentially much faster from that aspect too.