I have not purchased or used any MS software since 1979 so I am no help there, but v.42bis can, in theory do 4 to 1 compression, the newer v.44bis in v.92 modems does 8:1 in theory
not a lot of serial ports run at 460,800 bps so they are not going to reach theory all that often. I tried to remain child-like, all I acheived was childish.