I'm curious about the performance compared to something like, say, the RTX 3070.
Low. Apple doesn't have matrix math accelerators in their current GPUs. The neural engine is small and inference only. It's also only exposed by a far higher level interface, CoreML. Where it could still make sense is if you have a small VRAM pool on the dGPU and a big one on the M1, but with the price of a Mac, not sure that makes a lot of sense either in most scenarios compared to paying for a big dGPU.
Why is it inference only? At least the operations are the same...just a bunch of linear algebra