I think we are all over the place about speed because we are making unspoken assumptions about performance over cost. Here are a couple of real life examples. If an em runs on hardware with the same kind of profile as the HD 4870 (first graph), the economically optimal choice will be to run as fast as possible; if it's more like the GeForce GTX 260 (second graph) it will prefer to work at the lower "sweet spot" rate.
As long as we don't know what the hardware is like, we can imagine all sorts of profiles, including weird, monotonically decreasing ones. How much any of that will matter depends on how steep the graph is, i.e. on marginal cost. Larger slopes will cause more herding at the optimal rate. If I had to bet, I would bet on that kind of tight range, with ems down-clocking only if idle and up-clocking (if at all possible) only to cope with temporary spikes in workload too small to warrant delegation. But, who knows what the real profile will be like.