For end-to-end tests I user SuperAgent (
http://visionmedia.github.io/superagent/) or more specifically SuperTest (
https://github.com/visionmedia/supertest) which has a suite of assertions built in.
It actually tests your API over HTTP, so you can tell it the url (localhost or production) of any api you wanna test. Use it in conjunction with any modern test framework and you'll likely get the request-times/performance metrics echo'd as part of the runner's output.
It's typically used more for javascript development, but there's no reason that it couldn't test an API written in PHP (or anything else), as long as you're comfortable writing your tests in javascript.