The big difference is that CPU cores are truly independent. GPU "cores" aren't, a group must be running exactly the same instructions at exactly the same time,

The big difference is that CPU cores are truly independent. GPU "cores" aren't, a group must be running exactly the same instructions at exactly the same time, and they share registers. Also, CPU cores tend to have speculative execution, GPUs don't. So long as GPU threads have to run in a warp, they will be fundamentally very different from a CPU core, no matter how weak, strong, or power efficient.