Surely the main advantage of copper interconnects is they're cheaper than doubling the number of wire bonds. The chips are bussed on the DIMM anyway so I can't see how shortening the signal path my a few mm is going to make a 40% power difference all by itself. There's going to be fractions of a puff less per bus line.
Are modern SDRAMs even wire bonded any more or aren't they more likely to be flip chip? I guess if you're stacking them the top one would have to have wires.