That's true. The Pentium 3 at 1GHz was faster than a Pentium 4 at 1GHz, but the Pentium 4 could clock far higher with it's new pipeline design. The end of their range it maxed out just under 4GHz or so, and we haven't seen much higher ever since (for example, my i5 3570K steps up to 3.9GHz 1-core load). The only thing that keeps pushing for more performance has been multi-threading and more efficient CPU's, with larger/better caches, more instructions to play with (if programs are enabled for them), etc.
An interesting dimension to add is power consumption per MHz. From that you could then calculate a performance/energy, as you have both Dhrystones/MHz, and mA/MHz, which divided on each other would give Dhrystone/mA ratio, or simply put "computing efficiency". That would be interesting for low power electronics like battery powered stuff which main driver is the MCU doing stuff on an regular basis.
I don't know if it's acceptable to take these figures from the datasheet.. it can depend a lot of what peripherals are turned on (ARM) or supply voltage.
I think I have a board lying around with a PIC32 on it. I will see if I can run the test on that too, see how well MIPS4k compares. They claim 1.65DMIPS/MHz on that.