In Piwik 1.0, you can already use server-side tracking, so there's no tracking code on the browser.
In Piwik 1.1, you'll be able to POST. The URI is simply http ://yoursite/piwik.php
However, contrary to what you've read, shorter URLs for the tracking request don't speed up the page load. Asynchronous script loading and requests to the server do have perceivable improvements to page load time (at the expense of backward compatibility with older browsers).
In the case of web analytics, a cookieless domain (ie a website that doesn't serve up cookies) defeats accurate visitor tracking. Also, the transparent pixel image that is served up isn't entirely static...the HTTP headers change.