I have implemented a, crude, but working 'complier' to turn Google Protocol Buffers into GWT ready
java. It uses the base from the google code project here ... http://code.google.com/p/protobuf-js/, which I translated to Java so it could be included and complied by GWT (I added the ability to parse floats as well).
To test I accessed ~3500 protocol buffers and parsed them on different browsers (each containing a String and ~7-10 floating point numbers). I found varying results...
IE: ~72,000ms (stellar)
... this result had to use event chaining as to not choke the browser (FF & IE).
With such variation in parse time, this was not a reasonable solution. Parsing the protobuffers server side and sending them in JSON turned out to be MUCH faster. With all browsers having a reasonable JSON parsing time (under a second).
For the optimal solution I just modified my 'compiler' to spit out GWT overlay classes for the JSON instead of the proto classes. I then send the JSON wrapped up in a GWT-RPC. All works great now, very snappy performance.