If you want to change the CCA threshold, then it can be done using the Contiki radio API. For example, to set the threshold to -80 dBm:

  NETSTACK_RADIO.set_value(RADIO_PARAM_CCA_THRESHOLD, -80);

If you need something else, clarify the question.

Thanks Elsts

Is there any scope to reduce contikiMAC timing constraints further like time spent on  CCA, ti,tc,td  etc...