Advertising copy abuses the maximum compression available to give the impression that huge throughput is available, in reality compression by the modem only helps with certain types of file and if your modem is smart enough it doesn't try and re-compress gzip and other compressed files as doing so makes them bigger. In which case a dose of compression before sending the file will do you more good anyway as gzip compresses better than modems. In reality, for most people and except for connections at dead of night, the limiting factor is likely to be the other end of your internet connection getting slow due to user demand. The way to really find out how fast a connection you have is to send or receive a large file and time how long it takes. Hope this helps.
Any corrections always welcome.