The serial port speed setting determines the communication speed between the serial port and the modem and, in order to take advantage of the full bandwidth of your modem connection that's using compression, your port speed should be set higher than the expected speed from modem to modem. However, the GeoPort modem isn't really using the serial port in the same way. If you read page 115 of the Express/Fax Modem manual that came with the GTAK I it states:
"Traditional modems-external modems or even modems that plug into a slot
on a main logic board-use an RS-232-like serial connection protocol
between the CPU (Data Terminal Equipment-DTE) and the modem (Data
Communications Equipment-DCE). Your modem does not use this type of
serial connection.
Since your modem has no DTE-DCE serial interface, commands related to
the serial interface have no effect, but still exist for script
compatibility reasons."
The fact that the other modems may set the serial port speed to 57.6 kbps indicates to me that the AICK team just chose that modem speed to put there.
On why the modems connect at higher than 14.4 kbps anyway. By default, modems will attempt to connect at the highest speed they are capable of (providing the other end can support the same speed). Therefore, even if the port speed is set to 14.4 kbps, the modem is still negotiating V.34 protocols. However, as you point out, it is desirable to have your serial port speed higher than your modem speed to take advantage of the compression (for traditional style modems).