I explained why. Your upstream and downstream are not independent. Use *any* program and do this on *any* cablemodem, and you will see the exact same thing. Upload a large file to a ftp server and then try to visit any website and download something... it'll be extremely slow.
FWIW, your upstream cap is most likely 128kbps, which is 16k/s. With TCP and DCC overhead, sends at 14k/s are probably the maximum you can achieve.
-chris
