Whoh there! Hold on a second. One quick question: How long did each test run for? And how much longer did the under-clocked test run? Also, where you writing said file to flash? If so, you're not testing MPU, you're testing flash write, since that's by far more expensive than the MPU running. And if you're not counting the extra idle time the system has after the faster version is done, it's not exactly a fair test. the whole concept of race to idle means you need to count that idle time as part of the equation.
for i in $(seq 1 9000); do md5sum sintel-trailer-480.mp4; done > /dev/null
All other things being equal, I'm willing to bet that a device clocked at 500-900 will have much better savings than one clocked at 250-600 (from my own experience).