Taken from the site : http://www.the-scream.co.uk/html/resources/adsltips.html
Check it out, as it goes on to a discussion about RWIN and MTU settings.
"GETTING THE MOST FROM YOUR ADSL CONNECTIONcable
>> TS! ADSL PROVIDERS GUIDE
The first time you connect to the Internet with your all-new ADSL modem, you may find that you are not surfing or downloading at the rate that you had hoped the connection would give you. This guide will hopefully help you to optimize your broadband connection.
Maybe you are unsure as to what downloading speeds you should be getting. Well, if you have a 500Kbps connection, you should be getting close to 60K/s when downloading. This does depend greatly on the site you are downloading from, but I have generally managed to obtain decent download speeds of 57-60K/s.
If you work it out: 500000 / 8 = 62500 (62.5K)
So 60K/s is pretty good really. In fact, the actual reason that you could never obtain a true 62.5K/s download is because of transfer loss. This can be as much as 13%. TCP/IP loss can be up to 3% and the rest is ATM loss, which can be up to 10% (ATM = a protocol for transferring data between two points).
If you're not getting this sort of speed then it is probably because your RWIN and MTU settings are incorrectly set for broadband optimization. There are other reasons of course, such as contention, but incorrect optimization is the most probable."
So with a 128 you should be getting 13.9KB, 256 = 27.8KB and so on.
I agree tho, you shouldn't be using bit-torrent downloads as a guide, it should be off a dedicated site