It doesn't work that way.    When you have multiple members in the same pool, the data is striped across the member in proportion to their relative sizes.   So more of the volume data will reside on the SATA member since it will likely have larger drives.   This is especially true is the SAS is at R10 and the SATA at R50/R6.   Which is pretty common.   In the 5.2.x firmware, if the arrays are pretty busy most of the time,  the balancer will swap busy blocks with idle blocks until they members have better balanced latency.  For short term transitory IO loads the balancer won't be active often.  You can set RAID preference on a volume basis, if the members are at different levels.  You can bind a volume to a single member as well.  However, best thing to start with is monitor the group with SANHQ.  This will help you figure out what kind of IO load you are putting on the group.   Plus if you have to open a support account sometime this data is also valuable to support. 
 Assuming I have a mixed pool of SAS and SATA, where do new writes land?  My concern is that writes happen on both SAS and SATA equally and then over time the the data on the SATA would be moved to SAS if it needs the high performance and if data on SAS is cold it moves to SATA. I'm leaning toward a configuration that initially writes everything to SAS assuming it should be high performance and then over time, move it down to SATA.  It seems that this should be possible. I'm looking for clarification and maybe a document that could help me understand how to configure my array to land everythying on fast and move down to slow over time.
SAS/SATA Mixed Pool and Initial Data Placement