Rich, given that fact that you have managed to hack your WDMBWE with ssh and you know about the uptime command I am assuming that you know a bit about computers.
There are so many factors affecting the transfer rate like source system disk speed, CPU, network,router etc not to mention WDMBWE's own performance.
I only bought mine last week so I am still playing with it, but I too have managed to hack it and get SSH working along with NFS.
I am currently frigging it to work with both windows (for the kids) and various versions of Linux for me. I am a little disappointed with performance but that could just be because I'm tight and I didn't want to buy a proper NAS.
Anyway to answer your quesion the best way to work out your transfer speed is to transfer something and time it. Create a big file >=1GBytes and copy it to the WDMBWE.
If you are using widnows:
echo 1234567890 > abigfile.txt
REM Create a 1Gb file
for /L %i in (1,1,26) do (type abigfile.txt » abigfile.txt)
REM Start the copy and time it
copy abigfile.txt %YOUR_WDMBWE_DRIVE%
If you are using Linux:
dd if=/dev/zero of=abigfile.txt count=2M
cp abigfile.txt /your_WDMBWE_mount_point
Subtract the two time and divide by 1GB and you have your transfer time.
FYI My setup is Belkin 54g wireless and a dual core CPU laptop. Windows perfmon reported 2.3MB/Sec MAX and an average of 2.1MB/Sec. Given that this was wireless 54g the maximum I could have expected to achieve was 54000000/8=6.5MB/Sec if I got full duplex and a perfect world but that is never the case so I guess 2.3MB/Sec will have to do.
While the transfer was in operation I used top on the WDMBWE to monitor it and it showed 34% wait time 3% user time and 24% system time with the rest being idle. However start 3 PC copying the contents of a SVD and the WDMBWE just grinds to a halt. CPU is maxed out and performance just dies. More on that when I have some specific figures.