If the Mirror server is dedicated to the mirror db and no other IO intensive process is running on it , then your guess logicaly make scence. But in larger time intervals the write/sec should be almost the same because the checkpoint is also applied on the principal db and all the dirty buffer pages are flushed to disk. So comparing the write/sec in a long enough period of time(long enough for a checkpoint to run on the principal) should result in relatively similar values.You can also use sys.dm_io_virtual_file_stats to capture the volume of writes on a specific file in different intervals.