That's what I thought, too, until I saw ARM's Hot Chips 2016 slides. Page 24 shows that they write transformed positions to RAM, and later write varyings to RAM. That's for Bifrost, but it's implied Midgard is the same, except it doesn't filter out vertices from culled primitives.

That makes me wonder whether the other GPUs with position-only shading - Intel and Adreno - do the same.

As for PowerVR, I've never seen them described as position-only shaders - I think they've always done full vertex processing upfront.

edit: slides are at https://old.hotchips.org/wp-content/uploads/hc_archives/hc28...