[drmpeg]

Taking the 2.8 kHz bandwidth limit into account, around 2000 to 2400 baud depending on the excess bandwidth of the root-raised-cosine filter used to shape the signal.

The bandwidth of a single carrier signal is symbol rate * (1 + RRC excess bandwidth). A typical excess bandwidth is 0.35, so 2000 syms/s * 1.35 = 2700 Hz.

You can use a tighter roll-off, like 0.2. 2400 syms/s * 1.2 = 2880 Hz. Unfortunately, the tighter roll-off increases the PAPR (Peak to Average Power Ratio) of the transmitted signal.

[mlyle]

Your comment is 100% technically correct, except that people reading it may think that baud means numbers of bits per second.

If you have more than 2 possible symbols, you can do much more than that; 256QAM could be getting 8 bits/second/Hz under very strong signal conditions.