We watch addEventListener to do the callback timing which jQuery uses under the covers. Any requests going through the browser's xhr interface will get instrumented and reported (this is what $.get uses).
Based on what you said, it sounds like enabling this feature will have no impact on existing front-end load times. What about error reporting... are you guys satisfied with where it's at now or do you guys feel like there is more accurate and more in depth js error reporting in the future?