I have a PowerEdge R815 with quad Opteron 6272 and a faulty DIMM slot at A2. The system still boots with no DIMM installed there, but with warnings of an invalid memory configuration. I can silence the warning, but what kind of performance penalty can I expect?
SnapOverflow
You obviously lose memory bandwidth. Also, the RAMless(?) socket will experience an increase in memory latency - latency-sensitive applications may suffer a significant hit. Less sensitive applications might run the same way as before.
What the actual impact is in your environment is pretty much impossible to say without testing.