If it can, then I'd like to see it happen. Even Intel's Celerons use more power than average ARM SOCs, while offering performance straight out of the 1990s.

CPUs from the N100 - Core 3 N355 lineup are more reasonable, but still fall behind by a large margin, and throttle easily (more on the N355 side). Whereas AMD is still struggling to produce anything that isn't a mobile heater.

What x86 chip offers both performance and power-efficiency comparable to, say, a Snapdragon 8 Gen 2?

Isa plays little role in power efficiency or heat.

This is completely incorrect, both in theory, and in practice, as we can clearly see. x86, as a CISC ISA, has been specifically designed without any power-efficiency considerations.

As opposed to ARM and RISC-V, which pretty much exist specifically to be power efficient, as is very common for RISC ISAs.

This is, quite literally, what distinguishes these two ISAs, and what made both of them happen in the first place.

If arm could match 1:1 it still wouldnt be viable. It'd need to be at least 10% more preformant for the same spec range.

Again, I'd like to see some reasoning or examples behind this.

Why would ARM need to offer better performance to be viable? It's already incomparably cheaper, more open, more power-efficient, and requires less cooling at comparable performance and power drain levels.

It is already superior in every category. What you're asking for is not only unreasonable, but also pointless and unnecessary.