As for SEO, nothing is measurable unless you are Google, and that is the point. Microformats help search engine in their mission of presenting 'cards' of information on their search engine as opposed to providing links; this may be less likely to send traffic to your web site, but maybe you are happy if you see links to buy your product at Walmart, Best Buy, Amazon, etc.
My caveat is that "fast" is an attribute that customers will notice, and JavaScript or not, every byte you send over the wire takes time. Some people think 50k of overhead to put metadata in an HTML document is a good trade off, other people try to fit the whole page into 5k, and their sites are fast in Africa, on an iPhone, wherever...