Neustar Web Performance is what BrowserMob has become. Like BrowserMob, scripts used for Load Generation or Monitoring are written in a custom Javascript Selenium implementation.
On-demand Load Generation makes a lot more sense in most situations than spending thousands of dollars on a commercial load generator. Even still, you can spend too much time and money learning the quirks of an API that you only use once or twice a year. We’ve written over 150 different scripts for over 30 different companies and am quite familiar with their API.