This blog entry may be of use. He does a great breakdown of the kind of bottlenecks heroku can run into, and how increasing dynos can help, and provides links and information to the official performance guide on heroku as well as some tools that will help you test your own app.
Worker performance really depends on how your site is built and what you are using them for. Background processing (image formatting, account pruning, etc) called Delayed Jobs is how you put them to work
EDIT // 1 March 2012:
Here is another blog entry that explored heroku latency and throughput performance for a variable number Dynos.
EDIT // 28 Feb 2013:
There have been some concerns raised in this post regarding Heroku’s random routing algorithm and how metrics may be misreported when scaling Dynos, specifically those provided by New Relic. This is still an ongoing issue and is something to note in the context of my previous answer. Heroku’s responses are linked within the post.
EDIT // 8 May 2013:
A recent post on Shelly Cloud blog analyses impact of number of dynos and web server used on application perfomance. Baseline performance script used there should be useful in performing further tests.