I don't recall if d3 uses SVG or canvas off the top of my head, I haven't done much more than toy with it. If it uses SVG, then yes, this could speed up d3 greatly.
It uses SVG, so it will speed it up a lot. Unfortunately for things like a force layout in a graph, the main bottleneck will be JavaScript calculating the positions.
WebRender doesn't accelerate SVG. It's certainly a potential future area to work on, though.
You're right, it doesn't at the moment. But the whole point behind WebRender is vector graphics are much better suited to GPU rendering - there's no reason SVG couldn't be targeted eventually.
EDIT: Realizing you are Patrick himself, of course you know this...