I am not sure what you mean here, since /help cps control doesn't pop up anything.
However, if you are referring to the -lN flag in the /dcc send command, that does work. When I do not use it my send to my self immediately shot way over 1000 kB/s. When I added -l5 (should limit to 5 kB/s) the send got stuck between 8 and 8.5 kB/s. Not sure why it's not stuck around 5, but there's probably a reason for it that when taken into account you do end up at 5...
