the data rate you will get will depend upon the lock time of the PLL (XR2211). this is typically set the by the lowpass filter time constant on the output filter, which is usually pretty slow. a better estimate would probably be closer to 250bps. always makes me impressed with 56kbps telephone modems. also, why is the audio bandwidth of the recorder so low? im assuming the 3kHz is the max frequency it can accept, and not the sampling rate, which means you can go higher than 1.5kHz (all the way to 3kHz).