I'm sorry, but you base this on what? I see a definite difference in line speed between our 1mb line at home and my 384 line at my flat on any online game I play. Even WoW.
I saw a difference as well going from 384k to 1024k.
Have a friend with 384k DSL and a 192k wireless connection will ask him how it plays. Although I am assuming he will route D3's traffic through the 384k.