I had a tour last Friday of the A-B phone support center at Mayfield Heights, OH, led by none other than the center director, Dave Clark. We went past the modem lab where the A-B kits (as well as a half-dozen other models I saw scattered across the bench) are set up and tested with line simulators.
I asked this very question, about what baud rate was reasonable. Dave said that with perfect-condition phone line simulators he could sustain 38.4 kb, but in the real world 9600 was the most reliable rate by far.
So set that sucker up according to the directions CD that comes with it and you are going to do as well as the phone line will let you.