Wouldn't it be logical to assume that K SKUs have better binned dies than non-K ones? So in theory you should get better efficiency at whatever power limit you decide to go for.
I don't think 7% is a significant cost saving when buying something that you'll use for years most likely, at least for a single PC. If you were equipping an office with dozens of them, then maybe.
At my current energy costs I have estimated about £36 a year in savings from the current measure's I place on my CPU. Big enough over multiple years to give me something meaningful I suppose, but also not great once I actually calculated it for this post.
A little rundown.
I have excluded the higher power savings under full load (which are higher) as I have yet to do any of that kind of load on my chip outside of testing. So to make it a fair comparison it wasnt considered.
I included the savings from undervolting, these exist at moderate loads and increase the higher you go.
I include the savings from my custom power schema's these actually provide a saving bigger than my undervolting in a lot of my use cases, in other use cases the undervolt provides the bulk.
I calculated based on an average 30w saving over 16 hours a day for 365 days of year.
What I categorised is schema settings includes my affinity adjustments. As an example if I block turbo clocks when watching media like youtube, twitch, netflix etc. especially full screen it saves about 20-30w. If the browser is using p-cores and I have all cores unparked it will use another 10-20w on top of that, so my media schema only has the 2 preferred p-cores always unparked, whilst others are not blocked from unparking the OS wont do so in this workload. Using software p-states in the schemas has an effect also.
If the only thing I did on my CPU was to lower the power limit, no undervolt, no custom schema's I probably wouldnt actually be saving much power unless it was really low like 35w, as in day to day use the chip will not often exceed 125w even at stock. Downloading at gigabit speeds on steam with p-cores, I expect is similar load to a heavily threaded game (steam really makes these intel chips ramp up and use a lot of power on high speed downloading) I have seen it clear 100w untamed. I dont put steam on e-cores though as then any game launched by it would inherit the affinity so I would then need to set manual affinity for every game.
At full pelt though my undervolt has a much bigger impact than 7% on CPU power, as a % of system power though it will be lower. But I dont compile software, software encode, or other similar use case on this machine.
For games, I also have custom GPU profiles that manipulate the clock speed the GPU will run at and the GPU is undervolted, that can save me 100w when playing a game, more realistically usually 50-80w. Combined with CPU savings probably exceed 100w. Some days I will be playing games all day, but I can go several days without playing any. I think if I factored this in more I am saving more than £36 a year. I not that long ago posted a afterburner screenshot on the map screen in dune spice wars in a thread, and it showed me instant dropping GPU by about 70w just from switching to a profile that caps my GPU clocks to 1500mhz, with no loss of performance.