The question comes on what are you trying to measure? Are you trying to measure the response time of your servers or are you trying to measure the weight of the client in the impact of response time? I put forward the hypothesis that it is possible to measure the client weight by examination of the times captured with the developer tools in both development and also in functional testing.
So much of this client weight is related to page architecture that if you are waiting for performance testing to show you that your page architecture is problematic then you likely will not have time to fix the issues and retest before going to production.
I also recommend the collected O'Reilly works of Steve Souders which will help to bring home the client bound concepts for page design and how much this impacts the end user experience over and above how fast the server responds.
http://www.oreilly.com/pub/au/2951