We haven't benchmarked against other autoscalers or done any direct comparisons.
Our main motivation was to write something that scaled as we scale manually (i.e. when we get a lot of traffic due to press hits etc, we scale up based on GA realtime data as well as New Relic response times). When we were given access to the GA live API it just seemed like a natural fit.
Just speaking off the top of my head here, but services like HireFire ping the Heroku API to poll for information on when to scale. The Heroku API is a little rate limited. You'll probably get a more responsive result using GA.
