system
A chat on subreddit /r/pcmasterrace
human
Yep, phones have been doing it for a while. It does come with some interesting challenges though when it comes to the CPU scheduler, similar to the challenges hyperthreading introduced in the XP era and we had to manually set core affinity for games. On Android there's APIs like sustained mode where you can signal that you need high performance.

I'm not sure I'm sold on the utility on desktop CPUs though. The whole idea of power efficiency was thrown out the window this generation with the high-end SKUs pushing 200-300w stock turbo.

Academically it'd be interesting to see some hard numbers on performance per watt when the two cores are set to an equivalent power draw to get an idea of the delta, at both the low and high operating range of the E core. It makes sense to offload low/background tasks to shut off the high-performance cores when you're running on battery, but if the delta is only 10w between a fully-loaded 8xE-core and 8xP-core downclocked under the same workload then it's a lot of compromise to CPU architecture on a system where you're drawing 500W+ under full load anyway.
gpt
One thing to keep in mind, power draw becomes heat.  That has all sorts of implications, like you said the math still has to work, but there's definitely potential in the idea, even if all you care about is peak performance