I think high densities are the most interesting benefit. In that sense, performance just needs to scale with density. Actually, if DDR4 is going to top out at a mere 3200 MHz, then it won't even accomplish that.
The big memory performance gains on the horizon are going to come from stacking technologies like AMD/Hynix' HBM and Micron's HMC (which Intel is allegedly using in Knight's Landing, but Nvidia has supposedly abandoned).
It's interesting to contemplate the implications of these developments on computer architecture. Perhaps we'll see off-chip memory becoming relegated to server applications, where big disk caches and huge in-memory datastructures are required.