The speed difference might just be because the modem was designed to work under windows only. Winmodem drivers for linux are still fairly experimental, and may not be up to par with their win-counterparts. Unfortunately that can the price you pay with cheap hardware.
http://linmodems.org/ might have some more info for you in the init strings, if you haven't checked there already.
Worst case, get yourself a proper modem.
Let us know how it goes!