Do you have any link that compares performance of workers to EC2 instance types (for a raw idea of how much power an existing app needs) ? Also, how does deployment to IIS server and running multiple custom executables for various other tasks workout on your platform.. I mean can a worker run multiple applications ?

We haven't made any direct comparisons, but AppHarbor web workers are very fast and can handle upwards of 200 reqeusts/second for cached responses of moderate size. For non-ASP.NET workloads, we offer background workers: http://blog.appharbor.com/2012/03/08/background-workers-in-b...