The bad news is that building the FIVR on die may hurt CPU temps and overclocking at higher clock speeds. According to leaked (but trustworthy) spec sheets, the upcoming HD 4770K series holds the line on base and turbo clock speeds compared to the 3770K, but increases TDP to 84W, up from 77W. That increase is likely driven by incorporating the FIVR. That doesn't mean the system's power consumption has changed, but a component that used to exist in a separate domain is now being cooled by the CPU heatsink+fan and sits within the same socket.
By spec, the 4770K has a 7W higher power ceiling than the 3770K. In this test, the difference in total system power consumption boils down to just 1-2W.
With IGP disabled, Haswell has nearly identical peak power consumption as Ivybridge (remember they are using two different motherboards)
But idle power and task energy test suggest a fair improvement in power efficiency