Dropping the rails router, controllers and rack in favor of something like webmachine and resources could potentially speed up rails and reduce the conceptual complexity as well as the stack depth.
I'm curious, have you looked at http://reneerb.com/? I contend as well that Rails is not well suited to REST. As it stands, I think Renee is about 2x faster than Sinatra.
Dropping the rails router, controllers and rack in favor of something like webmachine and resources could potentially speed up rails and reduce the conceptual complexity as well as the stack depth.