In WAAS, the oldest DRE and CIFS cache data ages out and is deleted when a partition full threshold is reached. This is also true for the /local1/ directory where the log files are kept, so if transaction logging is being done, it should delete the older files first when newer ones are being written. However, each cache aging algorithm is slightly different.
The WAE should be running at maximum efficiency when the disks are actually full and some aging out is happening. You should be getting the most cache hits at this point.
Hope that helps,
Dan