All M3 CPUs should have roughly the same DMIPS/Mhz. There will be some differences based on the speed of the flash memory used. NXP tends to use slower flash memory in order to get their excellent power consumption figures. However, the choice of compiler probably has a larger effect on the DMIP score than the hardware deifferences between M3s. Since I have that 1.25 figure everywhere, my guess is that it came from ARM and is taken under optimal conditions.
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