Apple is going all in on Metal systemwide, those GPUs are too weak to support Metal
As a small point, it's not so much a function of GPU power as it is the ability to understand the needed instructions for Metal.
Back in the 10.4 days, Apple started using Core Image although it wasn't a requirement(it sort of became a requirement in 10.6, but that's a moot point as all Intel Macs had a Core Image capable GPU). Some specific applications-like Aperture-did require Core Image.
All PowerMac G5s shipped with a Core Image capable GPU, but the Geforce 5200 shipped in a lot of the earlier ones was overall a pretty crummy GPU. Most lower end Intel Macs shipped with an Intel GMA950 GPU-an absolutely terrible GPU that none the less supports Core Image. A card like a Geforce 4Ti will kick the pants off a GMA950 in raw performance, but by the time OS X Leopard came around the speed improvements even from a low end CI-capable GPU were noticeable.
It's the same thing with SSE4 in Sierra-there are plenty of CPUs that beat a low-end compatible CPU in raw performance, but none the less just don't support the necessary instructions. Going back to my PowerPC experience, it's like running the retail release of OS X Leopard on a G3-it's just not going to happen despite the fact that I have G3s that are faster in terms of raw performance than G4s.
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