The CEC adds DSP resources to the BCM450 system. For digital trunks, digital sets and analog sets, you need DSP resources only for conferencing and for voicemail. That means for a mainly digital system, you probably won't need the CEC at all.
To see the actual usage of DSP resources, you can use BCM Monitor. The Media Card tab shows detailed usage of each of the 4 DSP devices on the BCM450 mainboard. The Usage Indicators tab shows the percentage of used DSP resources. BCM Monitor also allows you to log data in case you want to get usage info for a longer period of time.