- Embedded MegaRAID does not "need" a BBU, because it does not feature any cache that would need to be protected against power failure.
- Disk cache (Read it thoroughly! It's DISK, not controller!:-)) mode can be changed during normal operation without harming any data transfers.
- With those 7200rpm SATA drives, you will most definitely see an improvement in performance. Whether it suffices your requirements, is to be tested & verified.
- Of course, in case of a power failure, all data sitting in the disks' cache and not having been written to the disks' media will be lost. Even with a SAS RAID contraller WITH cache AND BBU, only the controller's caches will be protected. The disks' caches can only be protected by eliminating the risk of a power problem, e.g. with a UPS, and by keeping all fingers crossed that no controller problems will ever happen.
Sorry for mostly repeating Davie's comment, I was too slow.