SPDY is now enabled on the site, though I can't imagine there's going to be much of a performance difference.

It's really easy to do; just change your "listen" lines to:

listen	[::]:443 ssl spdy;
listen	443 ssl spdy default;

You may also want to add an extra header to confirm your server supports it:

add_header Alternate-Protocol 443:npn-spdy/2;

That's it; restart nginx and it should be working. If you use something like Wappalyzer, you should see the SPDY icon in the address bar.