When talking about the right tooling, I would also like to point to the (free) AJAX performance measurement tool (http://ajax.dynatrace.com/) and of course the (commercial) dynaTrace tool itself (both tools work perfectly together, giving you a complete picture of the complete end-to-end transaction and its bottlenecks, even in heterogeneous systems).

If you look at the dynaTrace site http://www.dynaTrace.com, you find a 2 minute demo, which is very interesting.

Although this sounds like a sales story, I have been using dynaTrace in practice now for 2 years, and the whole buzz on the website is really true; measurement of application performance in a depth that is useful for development, testing and operations (great dashboarding), which is so low on overhead that you even can run this in (pre-)production systems.