Hi,
since the C6678 DSP is limiting the payload size to 256/128 bytes, you have to uses these values regardless of the switch's capabilities.
Please note that there is a performance problem when a device writes to DSP memory using a payload size of 256 bytes:
http://e2e.ti.com/support/dsp/c6000_multi-core_dsps/f/639/t/158004.aspx
Ralf