FlashORB (who offers Flash Remoting server services) posted a benchmark comparison between SOAP and Flash Remoting in Flash MX 2004:
http://www.flashorb.com/articles/soap_vs_flash_remoting_benchmark.shtml
One of the interesting results is that SOAP calls end up using a lot more memory (perhaps for the lazy decoding holding up all the data). But even that, it’s still lacking in performance…hmmm…