> does the DMIPS per watt figure make any real statement
> about the relative power of each processor, or is it just
> a measure of efficiency?

Just imagine the inverse, i.e. Watts per DMIPS (where lower is better of course), which means how much Watts are needed to compute 1 DMIPS. This way it is more easy to understand what this measure of efficiency (there you go) in the shape of a combination of DMIPS and Watts means.
The relative DMIPS performance of the cores, on the other hand, is shown by the leftmost chart, as I explained already.

In case that with "power" you mean rather electrical power, not performance:

(1) 2.2 GHz e5500 has 1.76 times the DMIPS performance of 1.5 GHz e500mc.
(2) 2.2 GHz e5500 needs 1.32 times the Watts to compute 1 DMIPS compared to 1.5 GHz e500mc.

Results:
A) 2.2 GHz e5500 draws 1.76 * 1.32 = 2.3 times the power of the 1.5 GHz e500mc.
B) At the same clock, while being only 20% more performant, the e5500 draws 58% more power than the e500mc.