He didn't just proxy through a random "third-party," he sent everything via Contendo. If anything that sped the whole thing up unfairly. Contendo had (before Akamai killed them) a killer DSA product that basically sucked a webpage in via the closest Contendo datacenter then transported it over their optimized and compressed backbone to the Contendo datacenter closest to the user.
SPDY hits you a bit harder in setup costs to be able to sling lots of requests back and forth faster. This is great for someone like Google who serves 100% of the on page content themselves. Anyone with advertising, third party content, or using a CDN for delivery might want to do some extensive real world testing before bothering to implement.