That isn't really the same as in this case where we're talking about the same generation of memory. DDR3-1333 at C7 versus DDR3-1866 at C7 will yield a large improvement. C7 DDR3 is available through at least DDR3-2000 speeds, so it's certainly possible to scale bandwidth a lot more than you're suggesting. It looks like you can even get DDR3-2000 @ C6 these days.