There is also a Vulkan driver for the M1/M2 GPU already, used in Asahi Linux. There's nothing special about Apple's GPU that makes writing a Vulkan driver for it especially hard. Apple chooses to provide a Metal driver only for its own reasons, but they're not really technical.
Steam frame is more for streaming PCVR than running existing PCVR games natively.
For sure, you can squeeze a few percentage points more out if you optimize for TBDR, and there are some edge cases where it's possible to make TBDR architectures behave pathologically, but it's not that big a deal in the real world.
I also disagree that the Steam Frame is for streaming primarily. If it was, why put such a powerful SoC in it or using it as the prototype device for doing x86 emulation with Fex?
The Adreno 750 is a 3 TFlops GPU that _should be_ substantially faster than a PS4 or a Steam Deck. It'll play plenty of low-end PCVR games pretty well on its own, if Fex's x86 emulation is performant, which it is.
Like the Meta Quest 2, it's a crossover device that a lot of people will just use standalone.