I've tried numerous numbers to no avail. I don't think it's meant outside of fserves.....
Oh well, it'd be nice to have working one like I suggested that could be global or send specific. For example if I sent joe mycatandme.jpg which is only 750KB and will take no time I wouldn't cap it but if I'm wanting to send Jim MYHELPSYSTEM.PDF which is 15MB and I also want to play an online game (ie, I need my up/downstream) I can cap him at 2KB/s so my game is not laggy.
It'd just be a good thing, whether people use it for illegal files I doubt, because generally you don't want those to be slow.