Ok, I've actually implemented a test version for this. I added a configuration option:
I tested using firebug and found that the new method (deferred script) is slightly slower as it generates another round trip to the server... however, this is offset by a faster initial load of the page in many cases. Also, if the browser caches the script callback (since the hash code will only change if the script changes), it will further provide benefits upon repeated visits to the page.
Please take a look at the beta version from SVN and let me know what you think.
p.s. It is also possible to remove the xajax_core.js load validation script by setting the delay time to zero. This would leave only the <script> tag for the xajax_core.js and the <script> tag for the callback to the request URI.