Stefan Tilkov posted an article about the discussion about Java SOAP stack performance on InfoQ. It seems that some developers of the three major Java SOAP stack projects, Axis2, Xfire, and JAX-WS, are really concerned about the topic. Although part of it was not so comfortable, the discussion can still help to improve the development of those projects. I think Steve did tell the essentials of SOAP performance that developers need to care about.
I have tried to learn all the three frameworks. To me, the learning curve and tools are also important for an open-source project to be accepted by the developers besides code reliability.