Tried to find where you pulled this number. This study seems in-line with your hypothesis (it states a gain over HTTPS, and a loss over HTTP): http://www.guypo.com/technical/not-as-spdy-as-you-thought/

I believe it has been Google all along, I guess when you do things at their scale this matters, but indeed I can't see any average dev rushing to support it.

It depends. I have a site with a lot of assets (about 70 images) and using SPDY made it 30% faster than plain HTTP thanks to better pipelining.

It's lower overhead for the server, too.

Do you know how much lower overhead?

Well, it's not just the front end; it's the load balancer and whatever other infrastructure, since it packs things into a single connection more effectively. For hn, not an issue (since it's a single nginx in front of a single threaded arc process).

http://www.chromium.org/spdy/spdy-best-practices seems legit.

Have you measured your gains?

I believe some pron sites use it. The benefit increases with pages which have a lot of assets, which applies particularly well to typical pron sites with their multitude of images per page.