Andy, thank you for the hint! I was somehow under the impression that I need to adhere the 10 to 12 bits distance in both ways which seems kinda silly, because it would introduce a unnecessary gap between transmission to the CMX981. I guess this simplifies my life a little :)

Concerning FSB vs. CBUS: I already have a CML (PE0002) device connected via CBUS to CMX981 which supplies the transmission clock signal. The connection is not reliable for clock speed greater than 2Mhz. Additionally, I always have to give the address before sending data or receiving data. On plus side, it is SPI compatible. Later I may use the gumstix SPI port to connect to the CBUS port and use it simply for configuration. As you can see in the CMX981 block diagram, the device has one CBUS port switchable to FSB and two additional FSB ports. Using the FSB port 3 and enabling TX and RX data paths in the CMX981, I am able to simply send data to device without a register address and the device immediately processes the data according to the modulation scheme and delivers baseband I/Q data to another device (or loops it back to the gumstix). As I wrote before, the serial clock for FSB transmission can be set up to 9.126 Mhz.

I am already in contact with CML and they suggested to use the OMAP's McBSP for controlling TX and RX data, that's why I started this thread initially. 

So I am off reading Linux Device Drivers.