That's about right... speedtest websites usually report your speed in kilo BITS per second (thousands of bits per second).
Browsers and download managers report speed as kilo BYTES per second (where 8 bits = 1 byte).
So really... 6600 kb/s divided by 8 = 825 kB/s, which is about the same as what you see on a single threaded download from Rapidshare.
Download managers open up several connections to download the same file, therefore you normally end up getting a faster download rate anyway, so everything looks to be in pretty good order for your sync speed