The main impact in speed of SOAP vs. REST has not to do with wire speed, but with cachability. REST suggests using the web’s semantics instead of trying to tunnel over it via XML, so RESTful web services are generally designed to correctly use cache headers, so they work well with the web’s standard infrastructure like caching proxies and even local browser caches. Also, using the web’s semantics means that things like ETags and automatic zip compression are well understood ways to increase efficiency.
..and now you say you want benchmarks. Well, with Google’s help, I found one guy whose testing shows REST to be 4-6x faster than SOAP and another paper that also favors REST.