Steams Linux support (proton) is forked from wine. So no, it doesn't emulate.

Performance can still be an issue, mostly because the Nvidia graphics drivers are performing worse on Linux vs on Windows.

While it's not conventional emulation, my understanding is that the DirectX 9-11 support isn't very far removed from how shader recompilation works in modern console emulators (RPCS3, Xenia, Cemu, Yuzu, etc.). Steam now even distributes shader archives among Proton users so that they can be translated in advance instead of on-the-fly, which is something that Cemu users had been doing among themselves for a while.

The question then becomes if extra 100$ worth of hardware balances out these issues. For 500$ machines it clearly does, but I don’t know about the 1000$ price point.

Wine (and proton) is a reimplementation of windows APIs. You can call that emulation but that would just further dilute the word.

For example, it would mean every graphics card driver that supports opengl is an emulation of opengl.

Alright, emulation was the wrong word to use. Still, the performance on Proton is noticeably worse compared to playing on Windows on the same hardware, at least for a decent chunk of games. I'll generally get similar FPS with a lot more GPU/CPU usage or overall lower FPS.

Wine is not an emulator (scnr)