If I am using a 2-byte data path, can I forego a DCM and drive the USRCLK and USRCLK2 inputs with a REFCLK input?
A DCM is used to provide clock skew compensation between TXUSRCLK2 at flip-flops within the FPGA fabric and the REFCLK input at the external pad. This will be an issue if the REFCLK input is being used to clock data into or out of the chip.
If the REFCLK input is not being used for this purpose (i.e., if it is being used only inside the chip), it is possible to not use the DCM. However, we recommend that you use a BUFG if possible to minimize clock skew within the chip.