I am working on a RESTfull application which requires high scalability. I am considering Netty based frameworks for RESTfull applications. I went through some of the available options and tried to get what they can offer as a non-blocking implementation. Here are my findings:
If you really want non-blocking you need to do non-blocking from the ground up and have proper REST clients. Otherwise as stated in my comment the performance difference will be negligible and in many cases worse for NIO (Netty with thread sharing).
You should also know Tomcat and various other servlet containers that can work with JAX-RS support NIO. The issue is that even though NIO is supported it will still be a single thread per request. Only Play, Finagle, Vert.x and pure Netty (regardless of NIO) support a different shared threading model and thus have different mechanisms for doing concurrency.