"He goes on to say that the results he’s accumulated so far do not show major performance hits unless all modules are activated."

I suppose "major" is relative, but any usage of Jetpack modules at all not only greatly hurt speed for most users, but also SEO. It is a typical example of the lack of transparency behind a lot of the WordPress related applications and services out there. Not only does Jetpack (Photon) upload your images to Automattic-owned servers, but they don't even explain that is happening during activation, not to mention that it also kills your on-site SEO; it remains a "hacky" app for clueless users.

Besides the lack of transparency on features, there is also no disclosure of the server "map" that Jetpack is powered by. While a website and user base near Dallas (e.g.) might not see a speed decrease, 90% of internet users around the world probably will.

The fact that Jetpack, and other Automattic-owned plugins, continue to be "featured" and "recommended" with hype marketing across the WordPress universe just makes other developers more jaded and less interested to contribute to the community. It is definitely a cash cow for Automattic, but it is also their worst enemy as well.