Well the bitrate you see on the web player (if you hover over where it says Streaming or Optimizing to the top left corner of the video) would be the same thing surely? Or bring up the stats window - on Windows it's ctrl + click but on the Mac there are two keys you press when you click but I can't remember off the top of my head what they are. I'll try and remember to look later and post back.
Or do you mean the theoretical maximum speed of the connection from the location of the Slingbox to the location you are viewing from? Ie taking account of the routing etc?
I have a Mac mini setup at my Slingbox location and I installed the mini version of Speedtest on it so I can measure, but I'm not sure whether the network protocol used for speed test makes it a valid comparison; as you posted in the other thread, you get a download speed from Japan (albeit not the exact location of your box) to the UK that suggests you should get more than your 1900kbps Slingbox stream.
I wanted to check if my Slingbox 350 in Japan really was uploading at around 1900Kbps or if it was uploading at a much higher rate. If it is uploading at a much higher rate then the problem would likely lie somewhere else, such as the latency we discussed.
I'll ask the person in Japan living in the house where the slingbox is to run some bandwidth monitoring software to check that the upstream really is around 1900Kbps.