Well higher clock speed usually require higher voltage which shorten battery life and increase heat. I actually know nothing about the architecture of this processor and how efficient it is at stepping (I'm presuming it has a dynamic clock). Anyways I wouldn't mind either seeing some data on the impact of higher clock (both perf and effect on battery life) and perhaps having a user option to set it low or high or something to that effect (I'm assuming the impact on battery life is significant but that might be a bad assumption on my part).
If all things were equal, you're right - no such thing as a free lunch - so running faster means using more juice and lots of it goes to wasted heat.
In this case (for Evo users) - it's using a more advanced manufacturing process where the stuff in the SoC (transistors, etc, etc, etc...) are smaller and closer together - lots smaller. Smaller stuff takes less power to run than the same stuff when bigger.
So - in this case, if you put an Evo side by side with an equivalent made with the newer process, the newer one will be more power efficient - enter the Thunderbolt. In that case, tho, they made the battery smaller, so on that one, there wasn't the big gain in advantage.
In this case, the silicon is made with the smaller manufacturing process (45nm vs. 65) _and_ the battery is bigger.
The SoC - the MSM8660 - in the E3D does indeed have a dynamic clock like the Evo (it's pretty much the rule now).
And - we tend to expect a dual-core to run any given process at lower speed than if run on a single-core (providing things are optimized sufficiently for dual-core use - we won't know that until it's here).
Downside - higher resolution. With more dots to update on graphics-intensive tasks, more processing is required at the end. More pixels also mean more control transistors on the display itself (by definition for LCDs).
There are just too many factors to know in advance if this will act same, worse or better than an Evo on power.
It could be way more power-efficient - or it could be worse - we'll have to wait and see.
