Once you start pushing the performance envelope with the ARM core, your performance-per-watt advantage will become less pronouced.
The ARM processor looks so good in your benchmark because it is really not asking much of the silicon at all. Increase the ARM processor's clock rate to 3ghz, and you will need to add:
1) More cache to prevent memory core starvation
2) More voltage to make the silicon transistors switch faster.
This will cause the ARM processor to create a LOT more heat.
The key will be to find an acceptable performance level that doesn't work the silicon hard so you don't consume a lot of power. x86 or ARM, it doesn't really matter.