According to the manual, the data rate on the USA-28x maxes out at 1Mbit/s. FreePPP's modems configuration panel tops the data rate at 230kbps (which is interesting, as Low End Mac's entry for the Duo claims the limit is 56kbps).
My next goal is to get the Duo into the wireless age with a pair of RS-422 to Bluetooth transceivers. (At $199/unit, maybe not soon...) ;D