Understood and no problem that dynamic content may not be compressible or cacheable in practical terms.
BUT the static piwik.js could very usefully be performance optimized, e.g. :
a) support for local or CDN caching with "expires" (one year) and "etag" headers
b) file compression support with "piwik.js.gz" output + "Vary: Accept-Encoding" headers.
Not complicated, but surely a material performance boost?