I recently had some fun overclocking my old i5 4690.
IIRC, running the base frequency at 3.9Ghz instead of 3.5GHz, yield a very modest performance boost but added 20% more power consumption and temperature.
I then underclocked it to 3.1Ghz and the thing barely ran at more than 40°C under load and power consumption was super low! The performance was more than mediocre though...
Devil's Canyon and Haswell-E were great overclockers. I had an i7-4790K re-lidded with liquid metal stable at 4.7GHz all-core and (a bit later) an i7-5960X stable at 4.5GHz all-core. But yes power consumption and thermal output were through the roof.