Interesting! I believe that Sandra might have a network speed test, think it might rely on Windows files shares though. If you feel like running an FTP server, even temporarily, that would at least be one other (pretty lightweight) protocol you could try to push some large transfers through and see if there are any differences there. I have 0 experience with Windows home server, but if they didn't take out IIS from WHS, you should be able to run an FTP server through that, or try one of the free 3rd party FTP daemons that are kicking around.
Thanks for the update! Just an aside, but isn't that file transfer details feature in the file copy window in Win7 such a nice change of pace? Goodbye progress bar that says 30 seconds remaining for 10 minutes!