Certainly browser extensions are a smaller audience than web pages; but they do exist and I welcome any effort to standardize also the privileged interfaces that they use.
I just think it's ironic that due to privacy concerns these information should be sacrificed. The User-Agent includes everything&the kitchensink on information. From a technical point of view, all of the bits in the user-agent are of no practical use to make sites work better. Yet the two bits (gpu vendor and model) that would *actually* help making webgl apps run better, are withheld.
If somebody is going trough the trouble of identification of people by their user agent, they'll likely also sniff out webgl support and capabilities, flash version, ad-block settings, cookie-settings, local-storage settings and so on. For all intends and purpose, people are already uniquely identified GPU brand/model or no.