Is it really 2x or is it 2x at max load ? Since, as you say, you're not peggig the CPU - would be interesting to compare power usage on a task basis and the duration. Could be that the 3D cache is really adding that much overhead even to idle CPU.
Anyway I've never regretted buying a faster CPU (GPU is a different story, burned some money there on short time window gains that were marginally relevant), but I did regret saving on it (going with M4 air vs M4 pro)
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.